大牛请教987-214 so that /so as to

214    Climatic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the weather.

(A) so gradual as to be indistinguishable

(B) so gradual they can be indistinguishable

(C) so gradual that they are unable to be distinguished

(D) gradual enough not to be distinguishable（A）

(E) gradual enough so that one cannot distinguish them

想也想不明白怎么会是A?C为什么不行?

谢谢

虽然连小牛也不是，也回答看看。

系表结构用so...as to比较简洁，注意to后面的动作的发出者也是主语。