21. In recent years global warming is becoming concern for people all over the world.A. t

问题补充：

21. In recent years global warming is becoming concern for people all over the world.

A. the; the

B. /; a

C. /; /

D. the; /

答案：

A. the; the

(答案→)B. /; a

C. /; /

D. the; /

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