number (名词) n.数；数目
the number of…意思是“……的数字/数目”，介词of同其后名词构成介词短语，修饰the number.当它作主语时，谓语动词用单数。例如：
Many people took part in 10,000-metre race, but only a number of them kept on running to the end. 许多人参加了10000米跑，但只有一些人坚持跑到底。
A number of my friends think I should take a holiday.
我的一些朋友认为我应该休假。
The number of students is about twenty.
学生人数大约是20人左右。
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