In case of fire, stay low to the ground in a smoky room, smoke always rises towards the ceiling.
A. as
B. when
C. while
D. once
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(答案→)A. as
B. when
C. while
D. once
解析: 考察连词辨析。A因为,随着;C当…时;C当…时;然而;尽管;D一旦;句义:万一遇见了火灾,在有烟的房间里要弯腰到地面,因为烟总是向上到天花板的。根据句义说明上下文之间有因果关系,故使用as连接上下文。故A正确。