请教prep 。。。

gmatziyu

请教prep 。。。
74.        (29534-!-item-!-188;#058&004138)

Ina certain wildlife park, park rangers are able to track the movements of manyrhinoceroses because those animals wear radio collars.  When, as often happens, a collar slips off,it is put back on.  utting a collar on arhinoceros involves immobilizing the animal by shooting it with a tranquilizerdart.  Female rhinoceroses that have beenfrequently recollared have significantly lower fertility rates than uncollaredfemales.  robably, therefore, somesubstance in the tranquilizer inhibits fertility.
Inevaluating the argument, it would be most useful to determine which of thefollowing?
A.Whether there are more collared female rhinoceroses than uncollared femalerhinoceroses in the park
B.How the tranquilizer that is used for immobilizing rhinoceroses differs, if atall, from tranquilizers used in working with other large mammals
C.How often park rangers need to use tranquilizer darts to immobilizerhinoceroses for reasons other than attaching radio collars
D.Whether male rhinoceroses in the wildlife park lose their collars any moreoften than the park’s female rhinoceroses do
E.Whether radio collars are the only practical means that park rangers have fortracking the movements of rhinoceroses in the park
请问这题为什么选C呢？

切尔西

http://forum.chasedream.com/GMAT_CR/thread-449000-1-1.html#last
这个帖子有解释