题目

 If trees continue _______ at the present rate, it won’t be _______there is no trees for shade.    A. to cut down; long before        B. being cut down; before long     C. to be cut down; long before      D. cutting down; before long 答案：C如图所示，图中虚线是一跳水运动员在跳水过程中其重心运动的轨迹，则从起跳至入水的过程中，该运动员的重力势能 A．一直减小B．一直增大C．先增大后减小D．先减小后增大