燃煤锅炉在焚火前必须检查（），严禁缺水焚火。
参考答案：锅炉水位
从原点出发的射线在与TC曲线相切的产量上，必有（）
A．AVC最小
B．MC=ATC
C．ATC最小
D．MC最小
The news that a jet __________ Russian mountains killing 88 was one of the worst things I have known in the past five years.[ ]
A. crashed down
B. crashed into
C. met with
D. came across
