求教LSAT-15-4-7

The number of hospital emergency room visits by heroin users grew more than 25 percent during the 1980s. Clearly, then, the use of heroin rose in that decade.

7.     The author’s conclusion is properly drawn if which one of the following is assumed?

(A) Those who seek medical care because of heroin use usually do so in the later stages of addiction.

(B) Many heroin users visit hospital emergency rooms repeatedly.

(C) The number of visits to hospital emergency rooms by heroin users is proportional to the incidence of heroin usage.

(D) The methods of using heroin have changed since 1980, and the new methods are less hazardous.（C）

(E) Users of heroin identify themselves as such when they come to hospital emergency rooms.

这个答案非常不明白？答案C说造访医院急诊室的海洛因使用者人数是跟海洛因的使用发生率呈比例的，但是并不能完全支持结论,因为同样一个人可以反复使用海洛因然后反复去急诊室，也一样成比例。

相反，D说1980年后海洛因的是拥有了新方法，而且这种方法危险性比较低。这不正好可以支持使用增长的结论吗？

理由是，新方法的低危险度可以大大减少人们去急诊室的人数，1980去急诊室的海洛因使用者却增加了，则说明海洛因的使用的确是增长了。

请问D为什么不对？请帮忙看一下

According to your understanding, Option D is only an explanation of the result that the number of hospital emergency room visits by heroin users grew more than 25 percent during the 1980s.

Provided that the new method is much more dangerous, the result of it could be that the heroin uage keeps constant but the number of visits rises. If other varians,e.g. the number of heroin takers, change, the number of visits could change. For instance, if the number of heroin takers decreases dramatically while the more dangerous, new method of using heroin spreading among the heroin takers, the number of visits could keep constant or decrease.

This question provides a good example of explanation of a phenomenon. Usually the factors of a phenomenon are complex and multiple. Only in very few occasions, an effect has only one corresponding cause. Be careful.

An explanation is slightly different from an argument. Usually sufficiency is a standard for appraising an argument, i.e. the premise(s) of a good argument can sufficiently support the conclusion. But sufficiency is not necessary to an explanation.

Questions 6-7

The number of hospital emergency room visits by heroin users grew more than 25 percent during the 1980s. Clearly, then, the use of heroin rose in that decade.

7.     The author’s conclusion is properly drawn if which one of the following is assumed?

(A) Those who seek medical care because of heroin use usually do so in the later stages of addiction.

(B) Many heroin users visit hospital emergency rooms repeatedly.

(C) The number of visits to hospital emergency rooms by heroin users is proportional to the incidence of heroin usage.

(D) The methods of using heroin have changed since 1980, and the new methods are less hazardous.（C）

(E) Users of heroin identify themselves as such when they come to hospital emergency rooms.

STIMULUS MATERIAL的大意是

前提1：80年代里去急诊室就诊的吸毒患者增长了25%；

前提2：。。。(MISSING)

结论：总吸毒人数在80年代里也增加了。

题目是要求找前提2。

首先我认为D一定错，D说的是自80年以来吸毒人士改变以往的吸毒方式而采用了一种危险系数比较低的新方法。如果这是前提的话，就能推导出去医院就医的吸毒患者会减少。而这个结论跟前提1是矛盾的，也没法跟前提1一起推出那个结论. 。

其次C是对的。C说去急诊室就诊的吸毒患者人数是跟海洛因的使用发生率是呈一定比例的。这就是说，比如80年代前这个比例是50%，80年代里这个比例还是50%。而这个条件和STIMULUS MATERIAL里的前提1一起就能推出80年代吸毒总人数增加的结论了。这个跟你说的一个患者去反复就诊从而吸毒人员就诊率提高的情况并不矛盾，因为患者是否反复就诊是一个比例形成的细节，而无论这个比例是怎样形成的，反正最终结果是PROPORTIONAL。

thanks dpyxmg and 懒猫必胜。

重新看一遍题目，我发现之前还犯了一错误，就是把the use of heroin rose当作the user of heroin rose了，因此才一直想不通。

现在终于想明白，谢谢了。