题目

阅读下面短文，然后写出画线部分属于哪种基本句型。① High school can be a difficult time. However, ② clubs and other activities can make it much easier. No matter how well you may fit in, joining clubs, sports, or other after-school activities will be good for you. Here is why.Firstly, ③ after-school activities will help you develop your time-management skills. Because you have to plan ahead of time when to finish homework, achieve extracurricular ( 课外的) activities goals, and organise your social life, etc. Secondly, ④ they can also teach you how to work w如图所示，一长为L=18m的传送带水平放置，可以看成质点的物块静止于传送带的左端，物块与传送带之间的动摩擦因数为μ=0.2．（g=10m/s2）（1）若传送带突然以v=4m/s的速度顺时针匀速转动，求物块在传送带上运动的时间；（2）若传送带先以a=3m/s2的加速度顺时针匀加速启动，2s后做匀速运动．求物块在传送带上运动的时间．