题目

—Do you have any idea what Paul does all day? —As I know，he spends at least as much time playing as he ________. A. writes         B. does writing     C. is writing           D. does write 答案：B利用杠杆投掷石球，如图，作用在A点的力沿    时最小．已知石球重100N，OA：OB=1：4，则作用在A点力的最小值为    （不计杠杆自重及摩擦）．