求助NN大全693

693. The Audubon Society and other conservation groups, concerned over what they have perceived to be the serious threatening of the environment as posed by the policies of the government, are preparing for a major political effort.

(A) have perceived to be the serious threatening of the environment as

(B) perceived as the serious threat to the environment as

(C) perceive being the serious threat to the environment

(D) are perceiving as the serious threatening of the environment

(E) perceive as the serious threat to the environment

这个题我在B和正确答案E上挣扎了很久，最后还是错选了B：（

牛牛们帮我解释一下AS的用法，以及这个题为什么不能用as把！THX！