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

1.Such as to… 只表示结果状语：His manner was such as to offend nearly everyone he met-Longman

主句主语必须与不定式主语一致,且必须同动主动：gmat里未发现such as to不定式后用被动的例子。

2.Such…as to 
表示目的：只有唯一结构（in such a manner as to ; in such a way as to）

Xue Mei spoke in such a way as to calm us down.
The sales materials are presented in such a way as to encourage attendees to purchase the products on the spot.-From Beatgmat.

She was such a lovely girl as to win favour from others. 错

3.So as to…=in order to 表示目的状语：主句主语必须与不定式主语一致,且必须同动主动: gmat 里未发现so as to 不定式后用被动的例子。

I drove at a steady 50 mph so as to save fuel.

We went along silently on tiptoe so as not to disturb anyone.---longman

4.So…as to 表示结果状语（相当于so that的第二个用法）真题：

In the Renaissance, painters were so impressed with
Leonardo da Vinci as to ignore their own training and designate as a
masterpiece anything he painted.

主动优先于被动语态

when so that and so as to happens to having the same meaning,

then we should choose positive tense

thanks sharing