垂直刷新率又称为帧频或画面刷新率，帧频越高，图像的稳定性越好，PC机显示器的帧频一般在______Hz以上。
参考答案：85
The little boy ________ in the mud all morning. No wonder he is dirty from head to foot. [ ]
A. has played
B. is playing
C. has been playing
D. was playing
绞车滚筒上不得留有余绳，以免车过变坡点突然（）崩断钢丝绳。
A.减速
B.匀速
C.加速
D.变速
