采煤机滚筒升降动作缓慢的原因之一是：调高液压系统中安全阀损坏或调定值太低。
参考答案：对
The number of people invited ______ fifty, but a number of them ____ absent for different reasons.
A．were； was
B．was； was
C．was；were
D．were； were
25～50m以下脚手架基础做法
