so as to 的用法

climatic shifts are so gradual as to be indistinguishable at first from ordinary fluctuation in the weather.

A,

B. so gradual they can be indistinguishable

C.. so gradual that they are unable to be distingguished.

D. gradual enough not to be distinguishable.

E. gradual enough so that one can not distinguish them

俺选C，因为被动语态好像没有问题。但答案是A， 请各高人指点~

谢谢

不好意思，发错版了。

请斑竹删此帖。

谢了！