[求助]一道费费logic！

我GMAT刚刚起步，请教一道逻辑

70. A low-pressure weather system is approaching Plainville; rainfall results from about 70 percent of such systems in the LACE w:st="on">PlainvilleLACE> 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 LACE w:st="on">PlainvilleLACE>.

Knowing which one of the following, in addition to the information above, would be most useful for determine the probability that LACE w:st="on">PlainvilleLACE> will have a thundershower soon?

A. the percentage of thundershowers in LACE w:st="on">PlainvilleLACE> that occur in the spring

B. the percentage of spring rainfalls in LACE w:st="on">PlainvilleLACE> 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

答案 c，我怎么觉得b也正确，

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；

c是直接从低气压得出雷阵雨的概率，而b是间接得出，先得出下雨的概率，再根据百分比得出雷阵雨的概率。谢谢指点！

个人理解这个题的关键在于Soon这个词，这个词表示要得到的概率只是当前由低气压导致的降水的成为雷阵雨的概率。因此C是最直接的答案，直接说明低气压能够导致的雷阵雨的概率。

B是说明在该地区整个春季降雨中，有多少是雷阵雨，按楼主的算法再乘以70％，得出结果是有点不太合逻辑，也不十分准确。

这道题好绕啊,我看了半天才做出来.ADE的错就不说了.

答案C说,知道P地区从低气压而来的THUNDERSHOWERS的比例X,而低气压是肯定要发生的事情了.那么如果X很大,那么发生THUNDERSHOWERS的可能就很大,否则就很小.

选项B说,知道SPRING RAINFALLS里有多少是THUNDERSHOWERS,我们只能得到由SPRING RAINFALLS得到THUNDERSHOWERS的可能性,可能THUNDERSHOWERS是有很多很多种其他原因得到,由RAINFALLS得到THUNDERSHOWERS只是其中一个非常小的可能.那么我们怎么判断出发生THUNDERSHOWERS的可能有多大呢?

我表达能力差,希望你看懂了.

再说两句,举例说,如果THUNDERSHOWERS这种东西,100次只有1次是RAINFALL造成的,剩下99次都是我DASHASHA造成的.你把RAINFALL造成的比率搞得再明白也没用啊.你得搞清楚我DASHASHA什么时候喜欢搞THUNDERSHOWERS来玩啊.