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

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：选A考查状语从句。句意：空气污染越来越严重，所以我们必须趁现在还来得及的时候采取行动。根据句意可知，应用before，表示“在……以前”。