问题补充：
—students took part in planting trees on March .12 this year.
— Yes. It is said that the students is 300.
A. The number of ; a number of
B. A number of; the number of
C. The number of; the number of
D. A number of; a number of
答案：
A. The number of ; a number of
(答案→)B. A number of; the number of
C. The number of; the number of
D. A number of; a number of
解析：a number of一些，许多；the number of……的数目。句意：今年许多学生参加了三月十二日的植树活动。——是的，据说学生的数目是300.结合语境可知前文主语为复数形式，故系动词用复数。下文主语number表示抽象含义，故谓语动词用单数，选B。
