问题补充：
There are students in the school . But I don’t know the girls among them .
A. a number of , a number of
B. the number of , the number of
C. a number of . the number of
D. the number of , a number of
答案：
A. a number of , a number of
B. the number of , the number of
(答案→)C. a number of . the number of
D. the number of , a number of
解析：本题考查the number of和 a number of区别。a number of为量词短语，表示“许多……”，中心词是这个短语所修饰的复数名词，所以作主语时，谓语动词用复数形式。the number of表示“……的数量”，后接复数名词，其中，中心词为number，因此谓语动词用单数。
