Sentence Correction

10. 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
i choose c.the answer it gives is a.
i remember og said so...as.. should be apply in negtive sentence.such as not so...as....;could de anyone help me ?why the A is right?

Even IMO C seems correct. Not sure why A would be correct?

if there is no grammaticlly wrong in A, i think we must choose A,because A is shoter and more consice than c.
i just wondering is the idiom so....as to be right here?

....i confuse not so.... as with so... as to....

A is the right one~