凝汽器真空提高时，容易过负荷级段为（）。
A．调节级
B．中间级
C．末级
D．复速级
参考答案：C
There are many reasons why some children have bad manners, ______ is the ill information on the Internet and in TV programme．
A．one of them
B．it
C．one of which
D．that
（）更容易嵌存磨料，且嵌得均匀牢固。
A.灰口铸铁
B.球墨铸铁
C.软钢
D.铜
