题目

Children in England mustn’t work until they are 13．They need to have a work permit（许可）to start working． The jobs teenagers can doThe hours teenagers（13 and 14 year olds）can work Delivering（送） newspapers   Manyteenagers will get up early to deliver newspapers to houses in their localarea before going to school．They are known as Paperboys or Papergirls． Babysitting     Lookingafter young children in their  home while their parents have gone outfor the evening is a popular job for teenagers，as they get money for watching children andtelevision all at the same t18．如图所示，a、b两个飞船在同一平面内，在不同轨道绕某行星顺时针做匀速圆周运动．若已知引力常量为G，a、b两飞船距该行星表面高度分别为h1、h2（h1＜h2），运行周期分别为T1、T2，则以下说法正确的是（　　）A．飞船a运行速度小于飞船b运行速度B．飞船a加速可能追上飞船bC．利用以上数据可计算出该行星的半径D．利用以上数据可计算出该行星的自转周期