Not easy as the problem of pollution is, no one should just stop trying.A. to solveB. to b

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(答案→)A. to solve
B. to be solve
C. solving
D. being solved
解析：考查固定结构。Not easy as the problem of pollution is    ,是as引导的让步状语从句，是一个倒装句，正常语序应是as the problem of pollution is not easy    结构，考查做某事不容易结构，即be not easy to do 结构，所以选A。