Hi,

How would you explain the answer to this question?

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

[spoiler]Answer: A[/spoiler]

Cheers,