会员制证券交易所的组织结构中不包括( )
A．会员大会
B．董事会
C．理事会
D．监事会
参考答案：B
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
如图所示，是某工地用的滑轮组匀速提升重量为4000N的物体，在重物上升
1m的过程中，拉力F的功率为2500W，此时滑轮组的机械效率为80%，
求：(1)滑轮组做的有用功 ▲ J．
(2)拉力F的速度 ▲ m/s．
