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

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主动优先于被动语态

when so that and so as to happens to having the same meaning,

then we should choose positive tense

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