----- 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
网友回答
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。