请教一道prep新破解版题

174.
            (28920-!-item-!-188;#058&003515)

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 distinguishable

E. gradual enough so
that one cannot distinguish them

OG上有很多类似的题，简单说下：

A： so adj as to-- idiomatic

B:  so adj so that-- inidiomatic

C:  so gradual that --意在强调“ shifts ”主动的，有意识的（are unable to ）不能被辨别，违背了原句中一种结果的自然阐述

D：改变句意，且enough to do (常见) enough to be--inidiomatic

E: enough so that -- inidiomatic 没有这种说法

在GMAT里面没有见过反例，曾在哪里见过一句话：没有“有能力被做掉”这种说法。