[求助]请问关于so as to 和so that 的区别

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

E）gradual enough so that one cannot distinguish them

选A，请问此题如何解释

最简洁