大全550，once

playirrrr

1.        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

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

这道题我始终不理解为什么选A，难道once放在句首有特殊作用，麻烦大家解释一下

Icanw

不是因为once的缘故，而是所有的选项中只有A的主语可以作once从句中 (be/became) commone throughout the Westerm plains的主语。尽管主句用了被动。。。

playirrrr

那C为什么不行呢？、

dudulee

the decline不能做前面句子的逻辑主语

playirrrr

前句的主语是black-footed ferrets 吗？

dudulee

以前在西部平原上很常见，逻辑主语除了是一种动物没有其他东西了吧

lichabrend

up