题目

完型填空阅读下面短文，从A、B、C、D四个选项中选出最佳选项，使短文完整、通顺。Do you know how to study better and make you study more effective (有效的)？We all know that Chinese students usually study very hard for a long  1 .This is very good, but it doesn’t  2  a lot ­­, for an effective student must have enough sleep, enough food and enough rest and exercise. Every day you need to go out for a walk or visit some friends or some nice places. It’s good for your study.     When you return  3  your studies, your mind will be refreshed (清醒11．某同学用如图甲所示的实验装置来“探究a与F、m之间的定量关系”．（1）实验时，必须先平衡小车与木板之间的摩擦力．该同学是这样操作的：如图乙，将小车静止地放在水平长木板上，并连着已穿过打点计时器的纸带，调整木板右端的高度，接通电源，用手轻拨小车，让打点计时器在纸带上打出一系列点迹均匀的点，说明小车在做匀速运动．（2）设纸带上计数点的间距为s1和s2．如图丙为用米尺测量某一纸带上的s1、s2的情况，从图中可读出s1=3.10cm，s2=5.50cm，已知打点计时器的频率为50Hz，由此求得加速度的大小a=2.40m/s2．