下列短文，从短文后各题所给的A、B、C、D 四个选项中，选出可以填入空白处的最佳选项。

    It's easier to go down a hill than to climb up a hill, so it is easier to have bad habits than to have good ones.

    Bad habits do not come 1. They come little by little while people do not notice their danger. Schoolboys pick up little bad habits in schools and in the streets. When they can't finish their 2 , they copy from their classmates. If they see older boys 3 , they also want to learn to smoke. When they grow older, their habits become 4 strong that they can no longer get rid of (除掉) it. From copying, they fall behind, and smoking is 5 for their bodies. At last they become worse and worse. How necessary it is that we get rid of the bad habits in the beginning!

(多选)美国宇航局计划2025年前载人登陆小行星，图为畅想登陆小行星的宇航员．为训练宇航员能在失重状态下工作和生活，需要创造一种失重的环境．在地球表面附近，当飞机模拟只在重力作用下运动时，就可以在飞机座舱内实现短时间的完全失重状态．现要求一架飞机在速度大小为v₁＝500 m/s时进入失重状态试验，在速度大小为v₂＝1 000 m/s时退出失重状态试验．重力加速度为g＝10 m/s²，下列说法正确的是(　　)

在横线上填上适当的数。

用 表示“1”，那么 是____

默写下列诗文中的名句

下列物体中不具有弹性势能的是（　　）