导致支气管扩张的主要原因是（）
A.肺发育缺陷
B.感染或支气管阻塞
C.支气管受牵拉
D.肺泡病变
E.遗传因素
参考答案：B
(2013·四川省乐山一模)The pupils were playing games in the classroom ________ a gunman entered and began shooting wildly at them.
A．while
B．when
C．as
D．before
信访人提出信访事项，一般应当采用（）形式。
A.书面
B.口头
C.录音
D.视频
