题目

Do you usually go to school________bike or _______ foot?        A.by;by         B.on;by             C.by;on             D.on;in 答案：C粗细均匀的棒的重心在它的________，球的重心在________，方形薄板的重心在________．一端粗，一端细的木棒的重心靠近木棒的________端．