climatic shifts

[sq720]

8. 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

Why not D but A

[cramya]

so X as to be Y is the right idiom

Not to be distigushable(D) is wordier than indistinguishable

[Mayur Sand]

can some body give clear explanation why (D) is incorrect, i am some how not convinced with reason mentioned in last post .

[thetrystero]

8. 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

My answer: A

"gradual enough" doesn't give a sense of extremity that warrants the "indistinguishable-ness".

C doesn't sound right.
B: looks like it's missing "that"