[求助]大全550

   Once common throughout the Western plains, black-footed ferrets are thought to have declined in number as a result of the poisoning of prairie dogs, their prey.

(A) black-footed ferrets are thought to have declined in number as

(B) it is thought that the decline in number of black-footed ferrets is

(C) the decline in the number of black-footed ferrets is thought of as

(D) that black-footed ferrets have declined in their numbers is thought to be（A）

(E) the numbers of the black-footed ferret are thought to have declined as

答案A，想问下C为什么不对？谢谢