Air pollution is getting more and more serious, so we must take action it is too late.
A. before
B. after
C. until
D. when
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(答案→)A. before
B. after
C. until
D. when
解析：句意：空气污染越来越严重，所以我们必须在它很晚之前采取措施。before 在……之前； after在……之后； until直到……为止； when在……时候。根据句意故选A。