求助牛人：一道关于概率的逻辑题！！！！！

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

我认为原文的意思是这样的：低气压时降雨概率是70%，低气压时会导致雷阵雨。

那么要想算出雷阵雨的概率，应该是低气压时的降雨率*降雨是雷阵雨的概率，这个相当于条件概率，因为不下雨就不能有雷阵雨，所以选B。而答案是C，答案上对C的解释是 低气压时的降雨率*低气压时的雷阵雨率，我觉得这个解释是矛盾的，因为你既然已经知道了低气压时的雷阵雨率还算什么低气压时的雷阵雨率啊，莫名其妙，直接就是结论需要的概率了啊，希望牛人给我指点迷津，谢谢

呵呵 这道题目当年考LSAT的时候也是印象深刻。

问了很多人，似乎XDF的老师也问过了。但是始终没有真的让人信服的选C的解释。

我还是支持B答案。因为可以解释通。

你现在在北美法学院吗？

The key problem should be base on "low-pressure systems", So I think C should be better than B, if there is low-pressure systems limition in B, then B is better than C.

What's your opinion?

我是这样理解的：

题中出现了三个信息：rainfall, system and thunderrain.其中已定因素是system, cuz题中提到：“A low-pressure weather system is approaching Plainville”。

B、C均可以推算出概率。但B推算必須要讓THUNDERRAIN与已知的SYSTEM间用RAINFALL桥接起来，而C可以让T与S直接联系。问题是问“would be most useful ”即最佳答案，理应选更直截了当的C。

what do u guys think?