（原创）献给和我一样搞不定if only, only if, unless的烤友

jenny0827

之前本人一看到if only, only if, unless就哆嗦，看了陈向东再加上自己的理解归纳后总算豁然开朗，现将我的总结贡献出来，也算对CD的小小贡献吧，如果已经有大虾总结过就烦劳版主删掉一下哈~

一、A是B的充分条件：A→B  只要有A就有B
如果A成立，则一定有B；只要A，就会B
英文表述：if A, B; if only A, B; whenever/whoever/wherever(=if) A, B

例题：
GMAT TEST A-7.
If A, then B.( A→B)
If B, then C. ( B→C)
If C, then D. ( C→D)
If all of the statements above are true, which of the following must also be true?
(A) If D, then A.
(B) If not B, then not C.
(C) If not D, then not A.( A→D=非D→非A)
(D) If D, then E.
(E) If not A, then not D.

GMAT TEST 3-12, 13.
Meteorologists say that if only they could design an accurate mathematical model of the atmosphere with all its complexities, they could forecast the weather with real precision.（即an accurate mathematical model是forecast with real precision的充分条件，只要有model，就会有forecast with real precision）But this is an idle boast, immune to any evaluation, for any inadequate weather forecast would obviously be blamed on imperfections in the model.
Which of the following, if true, could best be used as a basis for arguing against the author’s position that the meteorologists’ claim cannot be evaluated?
(A) Certain unusual configurations of data can serve as the basis for precise weather forecasts even though the exact causal mechanisms are not understood.
(B) Most significant gains in the accuracy of the relevant mathematical models are accompanied by clear gains in the precision of weather forecasts.
(C) Mathematical models of the meteorological aftermath of such catastrophic events as volcanic eruptions are beginning to be constructed.
(D) Modern weather forecasts for as much as a full day ahead are broadly correct about 80 percent of the time.
(E) Meteorologists readily concede that the accurate mathematical model they are talking about is not now in their power to construct.

Which of the following, if true, would cast the most serious doubt on the meteorologists’ boast, aside from the doubt expressed in the passage above?
(A) The amount of energy that the Earth receives from the Sun is monitored closely and is known not to be constant.
(B) Volcanic eruptions, the combustion of fossil fuels, and several other processes that also cannot be quantified with any accuracy are known to have a significant and continuing impact on the constitution of the atmosphere.
(C) As current models of the atmosphere are improved, even small increments in complexity will mean large increases in the number of computers required for the representation of the models.
(D) Frequent and accurate data about the atmosphere collected at a large number of points both on and above the ground are a prerequisite for the construction of a good model of the atmosphere.
(E) With existing models of the atmosphere, large scale weather patterns can be predicted with greater accuracy than can relatively local weather patterns.

LSAT TEST 4 SECTION I-21.    
Whenever a major political scandal erupts before an election and voters blame the scandal on all parties about equally, virtually all incumbents, from whatever party, seeking reelection are returned to office.（即political scandal and voters blame是all incumbents returned to office的充分条件）However, when voters blame such a scandal on only one party, incumbents from that party are likely to be defeated by challengers from other parties. The proportion of incumbents who seek reelection is high and remarkably constant from election to election.
If the voters’ reactions are guided by a principle, which one of the following principles would best account for the contrast in reactions described above?
(A) Whenever one incumbent is responsible for one major political scandal and another incumbent is responsible for another, the consequences for the two incumbents should be the same.
(B) When a major political scandal is blamed on incumbents from all parties, that judgment is more accurate than any judgment that incumbents from only on party are to blame.
(C) Incumbents who are rightly blamed for a major political scandal should not seek reelection, but if they do, they should not be returned to office.
(D) Major political scandals can practically always be blamed on incumbents, but whether those incumbents should be voted out of office depends on who their challengers are.
(E) When major political scandals are less the responsibility of individual incumbents than of the parties to which they belong, whatever party was responsible must be penalized when possible.

二、A是B的必要条件：B→A  只要有B就有A
A对B是必要的；只有A，才会B；没有A，就没有B（实际上是B→A的逆否命题）
英文表述：A is necessary to B; only if A, B; not B unless(=if not) A

GMAT TEST 13-16.    
For a trade embargo against a particular country to succeed, a high degree of both international accord and ability to prevent goods from entering or leaving that country must be sustained. A total blockade of Patria’s ports is necessary to an embargo（即只要有embargo，就有total blockade）, but such an action would be likely to cause international discord over the embargo.
The claims above, if true, most strongly support which of the following conclusions?
(A) The balance of opinion is likely to favor Patria in the event of a blockade.
(B) As long as international opinion is unanimously against Patria, a trade embargo is likely to succeed.
(C) A naval blockade of Patria’s ports would ensure that no goods enter or leave Patria.
(D) Any trade embargo against Patria would be likely to fail at some time.
(E) For a blockade of Patria’s ports to be successful, international opinion must be unanimous.

LSAT TEST 2 SECTION IV-15.    
In a large residential building, there is a rule that no pets are allowed. A group of pet lovers tried to change that rule but failed. The rulechanging procedure outlined in the building’s regulations states that only if a group of tenants can obtain the signatures of 10 percent of the tenants on a petition to change a rule will the proposed change be put to a majority vote of all the tenants in the building(即只要有majority vote of all the tenants，就有obtain the signatures of 10 percent of the tenants，在此是解题的关键哦). It follows that the pet lovers were voted down on their proposal by the majority of the tenants.
The argument depends on which one of the following assumptions?
(A) The pet lovers succeeded in obtaining the signatures of 10 percent of the tenants on their petition.
(B) The signatures of less than 10 percent of the tenants were obtained on the pet lovers’ petition.
(C) Ninety percent of the tenants are against changing the rule forbidding pets.
(D) The support of 10 percent of the tenants for a rule change ensures that the rule change will be adopted.
(E) The failure of the pet lovers to obtain the signatures of 10 percent of the tenants on their petition for a rule change ensures that the rule change will be voted down by a majority of the tenants.

GMAT TEST B-13.    
No nation can long survive unless its people are united by a common tongue（即只要nation long survive，就有its people are united by a common tongue）. For proof, we need only consider Canada, which is being torn asunder by conflicts between French-speaking Quebec and the other provinces, which are dominated by English speakers.
Which of the following, if true, most effectively challenges the author’s conclusion?
(A) Conflicts over language have led to violent clashes between the Basque-speaking minority in Spain and the Spanish-speaking majority.
(B) Proposals to declare English the official language of the United States have met with resistance from members of Hispanic and other minority groups.
(C) Economic and political differences, along with linguistic ones, have contributed to the provincial conflicts in Canada.
(D) The public of India, in existence sine 1948, has a population that speaks hundreds of different, though related, languages.
(E) Switzerland has survived for nearly a thousand years as a home for speakers of three different languages.

17.    Ronald: According to my analysis of the national economy, housing prices should not increase during the next six months unless interest rates drop significantly（即只要housing prices increase，就有interest rates drop，在此是解题的关键哦）.
Mark: I disagree. One year ago, when interest rates last fell significantly, housing prices did not increase at all.
It can be inferred from the conversation above that Mark has interpreted Ronald’s statement to mean that
(A) housing prices will rise only if interest rates fall
(B) if interest rates fall, housing prices must rise（逆否命题非A→非B=原命题）
(C) interest rates and housing prices tend to rise and fall together
(D) interest rates are the only significant economic factor affecting housing prices
(E) interest rates are likely to fall significantly in the next six months


三、A是B的充要条件：A←→B  只要有A就有B，同时，只要有B就有A
英文表述：If（充分）and only if（必要）A, B

LSAT TEST 3 SECTION I-12.
Leachate is a solution, frequently highly contaminated, that develops when water permeates a landfill site. If and only if the landfill’s capacity to hold liquids is exceeded does the leachate escape into the environment, generally in unpredictable quantities（即如果landfill’s capacity exceeded，就有leachate escape into the environment，只要leachate escape into the environment，就有landfill’s capacity exceeded，在此是解题的关键哦）. A method must be found for disposing of leachate. Most landfill leachate is send directly to sewage treatment plants, but not all sewage plants are capable of handling the highly contaminated water.
Which one of the following can be inferred from the passage?
(A) The ability to predict the volume of escaping landfill leachate would help solve the disposal problem.
(B) If any water permeates a landfill, leachate will escape into the environment.
(C) No sewage treatment plants are capable of handling leachate.
(D) Some landfill leachate is send to sewage treatment plants that are incapable of handling it.
(E) If leachate does not escape from a landfill into the environment, then the landfill’s capacity to hold liquids has not been exceeded.（逆否命题非A←→非B=原命题）

带标记的原文会更清楚一些，详见附件。

babyro

谢谢楼主，豁然开朗

jenny0827

不客气哦，有帮助就好~

FZCBMG

谢楼主！

magic88718

xiexie ~

alvinzqn

非常感谢~~~

wnc1006

仔细瞧瞧

louislyn

楼主威武！

lynnpahn

很好的总结

ak007

谢谢~！