21．In recent years global warming has become concern for people all over the world.A. the

问题补充：

21．In recent years global warming has become concern for people all over the world.

A. the, the

B. /,/

C. /, a

D. the, /

答案：

A. the, the

B. /,/

(答案→)C. /, a

D. the, /

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