调节阀副线的作用是（）。
A.防止故障停车
B.必要时人工用副线调节
C.便于进行更换或检修
D.以上三项
参考答案：D
The last few years ________ many disasters in and out of China, from earthquake to drought.[ ]
A. see
B. are seeing
C. have seen
D. saw
采用变径管有何作用？
