粗料石是表面加工成凹凸深度不大于（）的料石。
A．10mm
B．15mm
C．20mm
D．25mm
参考答案：C
One of the first foreign expeditions _______ Mount Qomolangma arrived in Tibet in 1921.
A．climbing
B．climbed
C．had climbed
D．to climb
煤气发生炉空气进口管道上必须设控制阀和( )。
A．防爆阀
B．逆止阀
C．放散阀
D．隔断阀
