[求助] three logic problems from prep! plz help!

25. (30261-!-item-!-188;#058&004144)

Twelve years ago and again five years ago, there were extended periods when the Darfir Republic's currency, the pundra, was weak:  its value was unusually low relative to the world's most stable currencies.  Both times a weak pundra made Darfir's manufactured products a bargain on world markets, and Darfir's exports were up substantially.  Now some politicians are saying that, in order to cause another similarly sized increase in exports, the government should allow the pundra to become weak again.

Which of the following, if true, provides the government with the strongest grounds to doubt that the politicians' recommendation, if followed, will achieve its aim?

(A) Several of the politicians now recommending that the pundra be allowed to become weak made that same recommendation before each of the last two periods of currency weakness.
(B) After several decades of operating well below peak capacity, Darfir's manufacturing sector is now operating at near-peak levels.
(C) The economy of a country experiencing a rise in exports will become healthier only if the country's currency is strong or the rise in exports is significant.
(D) Those countries whose manufactured products compete with Darfir's on the world market all currently have stable currencies.
(E) A sharp improvement in the efficiency of Darfir's manufacturing plants would make Darfir's products a bargain on world markets even without any weakening of the pundra relative to other currencies.

´the answer is b

i can't figure out the chain

37. (32356-!-item-!-188;#058&006101)

Capuchin monkeys in Venezuela often rub a certain type of millipede into their fur.  Secretions of these millipedes have been shown to contain two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate the capuchins.  The rubbing behavior is rare except during the rainy season, when mosquito populations are at their peak.  Therefore, the monkeys probably rub the millipedes into their fur because doing so helps protect them against mosquitoes.

Which of the following would it be most useful to determine in order to evaluate the argument?

(A) Whether the two chemicals provide any protection for the millipedes against their own predators
(B) Whether the type of millipede used by the capuchin monkeys in Venezuela is found in other parts of the world
(C) Whether animals other than capuchins rub insects of any kind into their fur
(D) Whether the only time the millipedes are readily available to the capuchins is during the rainy season
(E) Whether secretions of any other insects accessible to the capuchins contain chemicals that repel mosquitoes

the answer is d

why e is not right?

56. (34605-!-item-!-188;#058&007579)

Crops can be traded on the futures market before they are harvested.  If a poor corn harvest is predicted, prices of corn futures rise; if a bountiful corn harvest is predicted, prices of corn futures fall.  This morning meteorologists are predicting much-needed rain for the corn-growing region starting tomorrow.  Therefore, since adequate moisture is essential for the current crop's survival, prices of corn futures will fall sharply today.

Which of the following, if true, most weakens the argument above?

(A) Corn that does not receive adequate moisture during its critical pollination stage will not produce a bountiful harvest.
(B) Futures prices for corn have been fluctuating more dramatically this season than last season.
(C) The rain that meteorologists predicted for tomorrow is expected to extend well beyond the corn-growing region.
(D) Agriculture experts announced today that a disease that has devastated some of the corn crop will spread widely before the end of the growing season.
(E) Most people who trade in corn futures rarely take physical possession of the corn they trade.
the answer is d

why c is not sufficient. if there is too much rain, it will do harm to the crop!?

ding

by myself

thanks very much

seems it to be obvious