惯性有利也有弊，下列实例中属于惯性对人们有利的是 [ ]

A． 人从行驶的车上跳下来容易摔倒

B． 大雾天在高速公路上，由于一辆车突然停止造成几十辆车追尾

C． 汽车上的司机和前排乘客都必须系上安全带后，才能驶上高速公路

D． 体育课上推铅球时，铅球出手后不再受推力，却仍然可以向前运动

She spent a happy afternoon      her coins and stamps she had collected those years.[ ]

A. to look for   

B. worked on  

C. sorting out  

D. watch over