急求一题SC解答！

柿子大侠

climatic shifts are so gradual as to be indistinguishable at first from ordinary fluctuation in the weather.
A,
B. so gradual they can be indistinguishable
C.. so gradual that they are unable to be distingguished.
D. gradual enough not to be distinguishable.
E. gradual enough so that one can not distinguish them


我果断的选了C啊，看到答案A，怎么也想不明白了！求大神们指教

DUKB24

http://www.manhattangmat.com/forums/sc-climatic-shifts-t5980.html