So X as to [verb] is preferred over So X that Y?

Goal 760 is so bright that he will score a 760 in GMAT (cause - effect/result)
Goal 760 asked a good question so as to enhance his understanding of English Grammar. (purpose of)

purpose of climate shift was not to be indistinguishable but because of the shift it was indistinguishable.

between A and C, C takes the cake.

This is a GMATPrep question. The OA is A and not C.

A is more concise and clear in meaning whereas C is wordy with the usage of 'that they are unable to be distinguished'.

Goal760 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

Official Answer: C

Can anybody please explain that when do we use So X as to [Verb] and when we use So X that Y?
Is there any difference between the two?

X is/are so Y as to Verb => in this structure, Y is the measure enough that makes X to do something

So X that Y => So that express the consequence, or the cause and effect. In this sentence, the "gradual" is the cause makes the "climatic shift" cannot be distinguish.

That is my understanding. Please correct me if I'm wrong.

I am not an expert but I see it as
1. so .. as has a sense of inference..
2. so .. that has a simple cause and effect;

I think GMAC uses so..as in very specific purposes, where general public use more liberally to avoid a long clause after 'that' so..that... clause here;

Not all so..large ...are in the form of X refer to following link:

https://www.manhattangmat.com/forums/met ... t7170.html

It has example of so large that... it can do ..; just giving a reason; where as

so ..as in the below case: ( I am not sure I understand this very well)

he category 1 to 5 rating known as the Saffir-Simpson Hurricane Scale provides an estimate of a hurricane’s potential of destroying or damaging property, and is primarily determined from wind speed; a category 5 storm has wind speeds so high as to blow away small buildings, completely destroy mobile homes, and cause severe window and door damage.

https://www.manhattangmat.com/forums/x-i ... t6537.html