实验性研究所具有的特点有（）

A.可操纵性

B.样本量大

C.设立对照组

D.单纯抽样

E.随机分组

This is the first time I ______, so I feel very curious about everything here.   [ ]

A. have been to this city        

B. had been to this city  

C. came to this city            

D. come to this city