问题
______the students in our class is 50 and ______them study 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 number of
B: A number of ;the number of
C: The number of ;a number of
D:A number of ;a number of
正确答案是：
