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  3. Liquid Nitrogen Nitrogen is the most common substance in Earth’s atmosphere. In the Earth’s atmosphere, nitrogen is a gas. T

Liquid Nitrogen Nitrogen is the most common substance in Earth’s atmosphere. In the Earth’s atmosphere, nitrogen is a gas. T

admin2012-05-23  29
问题     Liquid Nitrogen
    Nitrogen is the most common substance in Earth’s atmosphere. In the Earth’s atmosphere, nitrogen is a gas. The particles of a gas move very quickly. They run around and bounce into everyone and everything. The hotter a gas is, the【1】the particles move.
    When a gas is【2】its particles slow down. If a gas is cooled enough, it can change from a gas to a liquid. For nitrogen, this happens at a very【3】temperature. If you want to change nitrogen from a gas to a liquid, you have to bring its temperature【4】to 77 Kelvin. That’s 321 degrees below zero Fahrenheit!
    Liquid nitrogen looks like water,【5】it acts very differently. It disappears as soon as it touches the floor because it rapidly boils and changes back to a gas at room temperature. A flower dipped in liquid nitrogen will shatter because all of the water in the flower【6】instantly. An inflated【7】placed in liquid nitrogen will appear to slowly deflate. The air isn’t leaving the balloon, though. It’s just taking【8】less space because it’s getting cold and its particles are moving slower. Once the balloon is removed from the liquid nitrogen, it warms back up and returns to normal.
    Although experimenting with liquid nitrogen is fun, we【9】need to be careful with it. We always need to protect our eyes by wearing goggles and our【10】by wearing gloves. After all, we wouldn’t want to freeze and shatter like a flower!
选项 A、often
B、also
C、always
D、still
答案D
解析
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