可伸缩金属或非金属风管软风管的长度不宜超过（）并不应有死弯或塌凹。
A.2m
B.5m
C.3m
D.4m
参考答案：A
生铁与钢的主要区别是______不同．百炼成钢（将反复烧红的生铁在空气中不断捶打，转化为钢）的化学方程式是______．
How did it that the car fell off the bridge into the river? [ ]
A. come out
B. come about
C. come across
D. come up
