滑车安装在固定位置的轴上，只是用来改变绳索拉力的方向，而不能改变绳索的速度，也不能省力，叫定滑车
参考答案：对
It's high time that we students ______ even harder at our lessons as the national entrance examination is coming nearer.
A．work
B．will work
C．worked
D．have to work
