— Why didn't you call him yesterday?  

— I        him ,but the line was busy.  [ ]

A. do call    

B. did call        

C. was calling    

D. had called

肝素的抗凝作用是()

A．使凝血因子灭活

B．增强抗凝蛋白质的抗凝活性

C．激活因子Ⅻ及血小板

D．除去血浆中的Ca²⁺达到体外抗凝

E．形成凝血纤维蛋白原