so...as to与so that

Often major economic shifts are so gradual as to be indistinguishable at fist from ordinary fluctuations in the financial markets.

A. so gradual as to be indistinguishable

B. so gradual so that they can be indistinguishable

C. so gradual that they are unable to be distinguished

D. gradual enough not to be distingguishable

E. gradual enough so that one cannot distinguish them

prep里面的新题，答案A，我选C...

我认为

so...as to与so that如果在主语问题上不会产生歧义，两者是没什么区别的。

open to discuss...

so the rationale to choose A but not C is that C is redundant?