在我国的某高原上，这里有湛蓝的天空，绵延的雪山，无边的草原，成群的牦牛，虔诚的喇嘛，同时这里也是我国生态环境最脆弱的地区，该地区农作物单产高的自然原因是（   ）

一短跑运动员在5s内跑完了50m，汽车行驶的速度是54km/h，羚羊奔跑的速度是20m/s，那么三者速度从小到大的顺序是（　　）

为保护领土安全，国家在钓鱼岛周边海域部署了航母；当舰载直升机起飞后，舰体会略微上浮一些，舰艇底受到海水的压强将____，舰艇受到的浮力____(均选填“变大”“不变”或“变小”)。

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Do you know how to study 1 and make your study more effective ( 有效的)? We all know that Chinese students usually study very hard for long 2. This is very good, but it doesn't 3 a lot, for an effective student must have enough sleep ,enough food and enough rest and 4. Every day you need to go out for a walk or visit some friends or some nice places. It's good for your 5.

    When you return  6 your studies, your mind will be refreshed(清醒)and you'll learn more 7 study better. Psychologists (心理学家) 8 that learning takes place this way. Here take English learning 9  an example. First you make a lot of progress and you feel happy. Then your language study seems 10 the same. So you will think you're learning  11 and you may give up. This can last for days or even weeks, yet you 12 give up, and at some point your language study will again take another big 13. You'll see that you really have been learning all along. If you get enough sleep, food, rest and exercise, studying, English can be very effective and  14. Don't give up along the way. Learn 15 you are sure to get a good result.