sairamGmat 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 . Why is it not C

C is wrong for the following reasons:

1. Climatic shifts can not be able. Person can be able.
2. He is able to solve the problem.
They are able to be sent to the tour.---NOT correct. A revised sentence should be there.

e.g., They are fit for the tour. etc.

A is correct for two reasons:

1. "So XX as to YY" is the correct idiom.
2. GMAT likes conciseness.

Answer is A.

Adding further to GMATPERFECT, C is very lengthy. A is short and concise.