问题补充：
There are books on the shelf. And them is about three hundred.
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：a number of一些，许多； the number of……的数目。句意：书架上有许多书。他们的数目大约是三百本。故选C。
