He is _________ ping-pong in the gym.[ ]
A. play
B. plays
C. playing
D. to play
答案:C
人类卵细胞中含有的染色体数是[ ]
A.44条+XY
B.22对+XY
C.22条+X
D.44条+XX
怎样正确使用马笼头。