Many people watched the boy _______ a yo-yo just now. [ ]

A. to play with

B. playing with

C. to play

D. plays

主要根据暴露状况来抽取样本的研究方法是

A．描述性研究
B．队列研究
C．病例对照研究
D．实验性研究
E．理论性研究