缺铁性贫血最常见的病因是（）

A.慢性胃炎

B.慢性肝炎

C.慢性溶血

D.慢性感染

E.慢性失血

Li Lei ______ in the classroom now. But he______ a moment ago. He ______ to the teacher's office. 　　[ ]

A. is, wasn't, has gone 　　　

B. isn't, was, went 　　

C. is, wasn't, has been　　　

D. isn't, was, has gone