ACD
带电量为q的物体以某一初速沿电场反方向做匀减速直线运动,可判断电场力做负功,电场力为qE,克服电场力做功为qES,A对;电势能增加qES,C对;合外力为0.8qE,合外力做功为-0.8qES,动能减少了0.8qES,D对;