— What do you think of the rules? 

— I think sixteen-year-olds should be allowed            sports after school. [ ]

A. to play

B. playing

C. played

D. to playing

教育实验设计的主要内容包括（）

A.研究假设

B.自变量、因变量、控制变量

C.实验处理

D.以上都是