题目

物体沿直线以恒定加速度运动,它的位移x与时间t的关系是x=24t﹣6t2(x的单位是m,t的单位是s),则它的速度为零的时刻是(                                  )                                                                                          A.      s  B. 2s   C. 4s    D. 24s                                                                                                                                          答案:B
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