宝典对第70题的解释如下：

70. A low-pressure weather system is approaching Plainville; rainfall results from about 70 percent of such systems in the Plainville area. Moreover, the current season, spring, is the time of year in which thundershowers, which sometimes result from low-pressure systems, are most likely to occur in Plainville.

Knowing which one of the following, in addition to the information above, would be most useful for determine the probability that Plainville will have a thundershower soon?

A. the percentage of thundershowers in Plainville that occur in the spring

B. the percentage of spring rainfalls in Plainville that are thundershowers

C. the percentage of thundershowers in Plainville that result from low-pressure systems

D. whether low-pressure systems in other areas are associated with rainfall

E. whether Plainville has more or fewer thundershowers each spring than do near by towns

参考答案：C

思路：文中给出两个条件，1是低压天气将到达，而70%的低压天气导致降雨，2是现在正是1年中雷阵雨最多的季节且雷阵雨有时也是由于低压天气导致；答案（C）给出了低压天气导致雷阵雨的比率，用70%乘以这个比率，就可以得到下雷阵雨的概率了。

A、B没能和低压天气联系起来，D、E为无关选项

我感觉答案是B，想了半天没想通是C。又查了，发现讨论的答案也是B。

请确认一下答案好吗？

B

原文：1。LOW-PRESSURE的70%可能性有RAINFALL

2。目前正是LOW-PRESSURE

所以，如果RAINFALL是thundershowers的可能性知道的话，就可知道thundershowers的可能性了

C错，C的意思为：thundershowers中有多少比例由low-pressure造成的。如果改为low-pressure有多大可能导致thundershowers，则为答案