① 用玻璃与石棉摩擦后，发现玻璃带负电

② 用玻璃与云母相摩擦，发现玻璃带正电

③ 用羊毛与云母摩擦，发现羊毛带负电  

④ 用羊毛与丝绸摩擦，发现羊毛带正电

⑤ 用木块与丝绸摩擦，木块带负电        

⑥ 用木块与硬橡胶棒摩擦，木块带正电

试根据小明实验中观察到的现象回答下列问题：

    We are warned not to waste time because time 1 will never return. I think it quite 2. What does time look 3? Nobody knows, and we can't see it or touch it and no 4 of money can buy it. Time is abstract(抽象的), so we have to 5 about it.

    Time passes very quickly. Some students say they don't have 6 time to review their lessons. It is 7 they don't know how to make use of their time. They waste it in going to theatres or playing, and 8 other useless things. Why do we study every day? Why do we work? Why do most people 9 take buses instead of walking? The answer is very 10. We wish to save time because time is 11.

    Today we are living in the 21^st century. We 12 time as life. When a person dies, his life ends. Since life is short, we must 13 our time and energy to our study so that we 14 be able to work and live well in the future. Laziness is the 15 of time, for it not only brings us 16, but also does other 17 to us. If it is necessary for us to do our work today, 18 us do it today and not 19 it until tomorrow. Remember that time is much more 20.

兰州高原夏菜具有很高的质量安全水平，在国家农业部进行的质量安全例行监测中，一直名列前茅，是我国著名夏菜外调基地之一，热销全国多个城市。但是，2016年出现“史上最严重”菜价暴跌，地头价不到往年的十分之一，菜农损失惨重。与此相对的是，最终消费者却没有明显感觉到夏菜价格的下降。据此完成下列问题。

洋葱是生物实验中常用的材料．它的叶分为管状叶和鳞片叶两种，管状叶伸展于空中，进行光合作用；鳞片叶层层包裹形成鳞茎，富含营养物质．请据图回答下列问题．

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