Sentence Correction

Often major economic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the financial markets

A)so gradual as to be indistinguishable
B)so gradual so that they can be indistinguishable
C)so gradual that they are unable to be distinguished
D) gradual enough not to be indistinguishable
E)gradual enough so that one cannot distinguish them

OA A
[spoiler]
I had read somewhere :
so gradual that is correct (however I did not choose C as it is awkward, i went for D instead)
as gradual as is correct.

so gradual as is incorrect
somebody clarify ?[/spoiler]