题目

Though Liu Xiang has retired(退役) from our national team, we will _____ him as before. A. keep up with                          B. depend on                            C. be proud of 答案：C如图所示，用一根硬棒撬起一个石块，棒的上端A是动力的作用点，若用最小的力撬动石块，请标出此时杠杆的支点O并画出最小动力F及动力臂L．