求助 prep里一道有关be able to 的语法题

alphaspace

相关的贴也看过了 不过还有些不太明白
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

解释中说C选项错的原因是C. able/unable to be done的结构错误

疑问是 如果把C 改成 so gradual that they are unable to be distinguishable  是不是就对了呢？

x110hp

我觉得 indistinguishable 是形容词，able to be 是+动词，
如果单纯的把distinguished 改成distinguishable不是把动词改为了形容词，
应该是不对的。