题目

Every day in China, about 200 million children go to school. Many of them take school buses. What should we do to keep these students safe on the road?On April 5, the State Council (国务院) published new rules about school bus safety. These new rules are much stricter than the old ones.Under the new rules, local governments must make sure that students attend nearby schools or boarding (寄宿的) schools to reduce traffic risks. Also, local governments must help kids in the countryside reach their school buses.From now on, school buses have a speed limit of 80km per hour on highways and 60一蹦极运动员身系弹性蹦极绳从水面上方的高台下落，到最低点时距水面还有数米距离。假定空气阻力可忽略，运动员可视为质点，下列说法正确的是（  ） A. 运动员到达最低点前重力势能始终减小 B. 蹦极绳张紧后的下落过程中，弹性力做负功，弹性势能增加 C. 蹦极过程中，运动员、地球和蹦极绳所组成的系统机械能守恒 D. 蹦极过程中，重力势能的改变与重力势能零点的选取有关