题目

质量分别为m1和m2的甲、乙两个物体，在同一斜面上甲保持静止，乙正沿斜面匀速下滑，则下列判断正确的是（    ） A.甲所受合力一定小于乙所受合力              B.甲、乙两个物体所受合力相同C.一定是m1小于m2                                D.甲所受摩擦力一定大于乙所受摩擦力 My mother advised me ________ a letter to her to explain it . [     ] A. to writeB. writingC. writeD. writes