Many cities issued a warning of air pollution for PM-2.5 readings which becameConcern of

问题补充：

Many cities issued a warning of air pollution for PM-2.5 readings, which became

Concern of all the people

A. the; the

B. the;/

C. /;a

D. an;a

答案：

A. the; the

B. the;/

(答案→)C. /;a

D. an;a

解析：第一个空，空气污染应是一个抽象的概念, 抽象名词前不用冠词；第二个空，表示泛指“关心的一件事”，故用不定冠词a。