What is Tom up to? He's seldom turned in his assignments these days, ________?[ ]

A. is he  

B. isn't he

C. has he  

D. hasn't he

调查研究中的客观性原则是指（）。

A.调查资料及结论不掺杂研究者主观因素

B.只承认’证实的’事实

C.把人看作是生理学意义的人

D.社会现象具有系统性和整体性