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