下图所示三位四通换向阀，该阀控制的是缸定式单杆活塞缸，当换向阀在中位时，活塞处于（）状态。
A.锁闭
B.浮动
C.差动
D.左移
参考答案：C
把图中的滑动变阻器连入电路，要求P向左移动时，灯L变亮．
Temporary failure is not to be feared. We must bear it in mind that the pains-taking efforts we have made will ______ in the long run.
A．pay off
B．take off
C．break off
D．work off
