题干

科学家在“哥伦比亚”号航天飞机上进行了一次在微重力条件(即失重状态)
下制造泡沫金属的实验。把锂、镁、铝、钛等轻金属放在一个石英瓶内,用太阳能将这些金属融化成液体,然后在融化的金属中冲进氢气,使金属内产生大量气泡,金属冷凝后就形成到处是微孔的泡沫金属。下列说法正确的是(    ) 

A:失重条件下液态金属呈现球状是由于液体表面分子间只存在引力

B:在失重条件下充入金属液体内的气体气泡不能无限地膨胀是因为液体表面张力的约束

C:在金属冷凝过程中,气泡收缩变小,外界对气体做功,气体内能增加

D:泡沫金属物理性质各向异性,说明它是非晶体

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答案(点此获取答案解析)

B

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用方框中所给单词的适当形式填空,(每个单词限用一次)。

feel   build  true   because   in frond of   behind   make  hard   a little   grade   sound     yes

       Is it good to be in the middle? Sometimes, no, like when you’re in the middle seat on a long car ride. But sometimes, ____, like when you’re in the middle of a great movie. What will happen next? Middle school is ____ bit like that.

     Middle school is called middle school ____  it’s in the middle of your school years. Elementary school is ____ you. High school and possibly college still await(等待) you. Middle school often includes sixth, seventh, and eighth ____, but you might go to middle school earlier or later, depending on how it's done in your area.

    For a kid, going to middle school is often a big change:

       ·First, it often means moving to a new ____, which takes some time to adjust(调整) to.

      ·Second, it may mean taking a different bus, with different students.

      ·Third, the friends you ____ in elementary school may end up going to different middle schools.

    All that can make you ____ a bit scared on the first day of school.

    Other things that probably will be different are the teachers and the work. Have you heard that middle school teachers are really mean and the homework is really, really ____? Oh, dear. We've heard those, too, but usually they’re not ____. Yes, you'll like some teachers better than others, but middle schools are not special for teachers!