pollution has become a worldwide problem,it may be many years ( )the situation improvesA.after B.before C.as D.since为什么

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pollution has become a worldwide problem,it may be many years ( )the situation improvesA.after B.before C.as D.since为什么
pollution has become a worldwide problem,it may be many years ( )the situation improves
A.after B.before C.as D.since
为什么

pollution has become a worldwide problem,it may be many years ( )the situation improvesA.after B.before C.as D.since为什么
B
因为逻辑上就是before.其余选项根本不足为虑.