再生障碍性贫血的病因是（）

A.脱氧核糖核酸合成障碍

B.珠蛋白合成障碍

C.红细胞膜的缺陷

D.造血功能障碍

E.红细胞被血清中抗体或补体所影响

—I knocked into a tree when I went to the bus stop for my friend.

—I suppose you _____too fast.

A．had driven

B．are driving

C．drove

D．were driving