请教so... as to 和so ...that有什么区别？

cyzmcl

新prep 里有一道题
174.    (28920-!-item-!-188;#058&003515)

Often major economic shifts are so gradual as to be indistinguishable at first 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 distinguishable
E. gradual enough so that one cannot distinguish them

正确答案是A，C为什么不可以呢？是因为so as to 和so that用法上的区别么？请NN赐教～

sj33333

在so...as to和so...that都可以得情况下，优先用so...as to，因为简洁的原则。so...as to因为to后面跟不定式，所以对其逻辑主语又要有，一般都是主句的主语。而so...that就没有这个限制。that后面是句子.

jason1618

在逻辑主语都相同情况下
C感觉太Redundance

cyzmcl

原来如此～～明白啦  thx～