--In recent years problem of global warning has been very serious.----That’s right

问题补充：

----- In recent years, problem of global warning has been very serious.

------That’s right. People are facing number of natural disasters now.

A. 不填;

B. a; the

C. the; a

D. a; a

答案：

A. 不填;

B. a; the

(答案→)C. the; a

D. a; a

解析：考查冠词。全球变暖的问题是特指，a number of，许多，为固定表达。