Sentence Correction

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


OA is A
When is the so..that idiom used?
whats wrong with using so...that here?
[Copied the question from a forum post to create a new thread

when struck b/n two correct idioms go for shorter one
c) its wordier to say unable to be distinguished, indistinguishable is better
a) correct idiom so(adj) as ...

Yeah, I picked A fairly quick on this one. None of the other answer choices shows me something better than A. A is correct because it uses the idiom so ____ as to ____.

Brothers and ladies,I wrote this post to ask for help.
Someone please answer my question.I just posted the SC to set the context.

mundasingh123 wrote: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


OA is A
When is the so..that idiom used?
whats wrong with using so...that here?
[Copied the question from a forum post to create a new thread

heard that Gmat prefers " cant" to " be unable" and the problem in C here is " unable to be distinguished" ( not allow to use passive voice here"

and heard that " so... that" is used to describe cause and consequence.... ( not sure much)
for example: The book is so interesting that I have read it many times
The water is so hot that I can't drink it