题目

The time and effort he has devoted during the past few years _______ trees in that remote area is now considered to be of great value. A. of planting    B. to plant   C. plant    D. to planting 答案：D三颗人造地球卫星A、B、C绕地球作匀速圆周运动，如图所示，已知MA = MB < MC，则以下说法中正确的是（     ）A．线速度关系为vA < vB = vCB．周期关系为TA < TB < TCC．向心力大小关系为FA = FB < FCD．半径与周期关系为