A number of students_____ playing on the ground but I don't know what the exact number______.

[       ]

A. are; is

B. are; are

C. is; is

D. is; are

若小明坐在爸爸的对面且与阿姨相邻，则哪一个人必须坐在妹妹的对面

A．阿姨

B．爷爷

C．奶奶

D．妈妈