a lsat question

21. 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 determining 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.

why choose b not c?