1.
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（A）

(E) gradual enough so that one cannot distinguish them

答案是A我没有问题，关键是C，我排出C的理由是A比C简洁，但是大全对C的结论是 C confusedly refers to the climatic shifts themselves as being unable when it is really people who are unable at first to distinguish climatic shifts。 我不知道这句话到底什么意思，请帮我解释下。