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阅读下面短文，掌握其大意，然后从以下各题所给的四个选项(A、B、C和D)中，选出最佳选项。
Why is a space left between the rails of a railway line where one piece joins the next? The space 1 to let the rail expand when it gets 2 . On a hot day, the metal expands and if it 3 to do this, the mils would bend 4 shape. This caused railway 5 in the past.
6 solids expand much less than liquids or gases, the 7 increases can add up to a lot, when there are long 8 of rail. For example, the difference in length of rail, in summer and winter, 9 the distance between London and Edinburgh, is about 1/4 mile.
We 10 keep in our minds and must not forget the expansion of metal when we build a very long mental bridge 11 . So spaces are left in the places 12 the mental is joined.
When 13 colder, it becomes smaller in size. We can make use of the 14 that solids expand when they are heated, and become smaller in size when they get colder and 15 they do it at different 16 .
For example, when an iron band has to fit tightly 17 a wooden wheel, first we make it 18 than the wheel, then we 19 it, so that it goes on easily. When it gets colder, it becomes smaller in size and 20 the wheel very tightly.
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