题目

It was three hours that the old man spent ________ the old clock. A. repair      B. repairing     C. to repair     D. in repair    答案：B 将重30N，底面积为10cm2的木块放在1m2水平桌面上，在木块上施加竖直向上的拉力F=20N，则木块对桌面的压强为（　　）A．5×104PaB．3×104PaC．2×104PaD．1×104Pa