There are lots of that students have to follow in school.
A：exercises
B：problems
C：skills
D：rules
矿井里的升降机，由静止开始匀加速上升，经5s速度达到6m/s后，又以此速度匀速度上升10s，然后匀减速上升，又经10s停在井口．求矿井的深度．
下列有关内、外力作用叙述正确的是( )
