问题
Your parents bought you _____ books last year， right—Yes. _________ them is 20.
A:the number of； a number of
B:the number of； the number of
C:a number of； the number of
D:a number of； a number of;
B:the number of； the number of
C:a number of； the number of
D:a number of； a number of;
正确答案是：
