Air pollution is getting more and more serious so we must take action it is too late.A. b

问题补充：

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

答案：

(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。