ZDF138

cz

[face=Verdana]#138) 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.

C) the decline in the number of black-footed ferrets is thought of
E) the numbers of the black-footed ferret are thought to have declined

选项C, E 错在哪里?  ( E ) 是不是因为 the numbers of  的 numbers 应该用 the number of ?  [/face]

Marrow

c,e的主语能符合once throughout..么？