问题补充：
the students in our class is 50 and them study very hard.
A. The number of; the number of
B. A number of ; the number of
C. The number of; a number of
D. A number of; a umber of
答案：
A. The number of; the number of
B. A number of ; the number of
(答案→)C. The number of; a number of
D. A number of; a umber of
解析：短语辨析。The number of……的数目；a number of若干；许多。句意：我们班学生的数目是50，他们中一些学习非常努力。故选C。
