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 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

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！