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

血浆胶体渗透压主要来源是（）

A.葡萄糖

B.Na⁺

C.K⁺

D.球蛋白

E.白蛋白