请问一道GMAT语法题

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 that they are unable to be distinguished

为什么选A呢？

A更简洁. as 后的主语很明确的指向climatic shifts.

be able to通常不用被动.

第一，第二句句型复杂（用了从句）。

第二，用了被动，过于复杂。

第三，有点多加了意思（...unable...）