题目

They are having a school trip. It's exciting! Let's __________________ them. A . join B . join in C . take part in D . to join 答案：A一火车以1m/s的初速度，2m/s2的加速度做匀加速直线运动，求（1）火车在第3s末的速度是多少？（2）火车在前4s内的平均速度是多少？（3）火车在第5s内的位移是多少？