问题补充：
More and more people have come to realize that if we had taken effective measures much earlier, the air quality so seriously now.
A. is not polluted
B. would not be polluted
C. had not been polluted
D. would not have been polluted
答案：
A. is not polluted
(答案→)B. would not be polluted
C. had not been polluted
D. would not have been polluted
解析：句意， 越来越多的人意识到如果我们早点采取有效地措施，现在空气污染就不会这么严重了。If条件从句是对过去的虚拟，故用过去完成时，而主句是对现在的虚拟，故用would do形式。
