prep逻辑一道，百思不得其解，请NN点拨

题目是这样的：
In a certain wildlife park, park rangers are able to track the movements of many rhinoceroses because those animals wear radio collars. When, as often happens, a collar slips off, it is put back on. Putting a collar on a rhinoceros involves immobilizing the animal by shooting it with a tranquilizer dart. Female rhinoceroses that have been frequently recollared have significantly lower fertility rates than uncollared females. Probably, therefore, some substance in the tranquilizer inhibits fertility.
In evaluating the argument, it would be most useful to determine which of the following?
a. Whether there are more collared female rhinoceroses than uncollared female rhinoceroses in the park
b. How the tranquilizer that is used for immobilizing rhinoceroses differs, if at all, from tranquilizers used in working with other large mammals
c. How often park rangers need to use tranquilizer darts to immobilize rhinoceroses for reasons other than attaching radio collars
d. Whether male rhinoceroses in the wildlife park lose their collars any more often than the park's female rhinoceroses do
e. Whether radio collars are the only practical means that par rangers have for tracking the movements of thinoceroses in the park.

答案是C，完全没懂，觉得都是无关。。。。
大家帮帮忙啦