题目

Most smokers say they _______ smoking to some degree but health experts suggest that they should give it up ______ their health.  A. benefit; for the benefit of      B. benefit from; with the benefit of  C. benefit; in the benefit of       D. benefit from; for the benefit of 答案：D 解析: 略一个小球在空中自由落下，机械能为45焦，当动能是15焦时，重力势能是________；在小球下落过程中动能________（填：变大、变小或不变），落地时重力势能为________.