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【答案】C【解析】【详解】A.根据曲线运动的知识，轨迹向合力的方向弯曲，由轨迹图可知，合力的方向先沿河岸向右后沿河岸向左，所以加速度方向发生变化，故A错误：BC.小船的速度先增大后减小，所以动能先增大后减小，合外力先做正功后做负功，故B错误，C正确：D.由于船头始终与河岸垂直，所以渡河时间为dt=V船相对于水的速度大小不变，则渡河时间不变，故D错误。故选C。

第四部分写作第一节短文改错It's safer to usepedestrian crossings or overheadbridge.Never cross a road by dashing across it,what isbridgeswhichinviting trouble.If there're no crossings,we must lookcareful right and left and cross only when that is safe tocarefullyitdo so.Some of us go to school by the bus.It's important forus not to getting on or off a bus while it is still move.Igetmovingtried to get on a moving bus once.It dragsme a shortdraggeddistance and nearly ran over me.I was lucky A escapetowith some scratches.Don't fool around in the bus and youorwill get hurt.