当燃油锅炉作为废气锅炉的汽水分离筒时，废气锅炉采用（）锅炉。
A．烟管
B．强制循环水管
C．自然循环水管
D．A或B或C
参考答案：B
I’m terribly sorry to have kept you waiting so long, but it’ll still be some time _________ Tony gets back.
A．after
B．since
C．till
D．before
