求助！！

请教一道XDF蓝皮书的题

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 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

我选的是C  答案是A

请问C错在哪里？ 谢谢

自己顶一顶

Thanks    i see