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  3. Suppose you have a brick and a sponge which are exactly the same size. The brick and the sponge will occupy the same amount of s

Suppose you have a brick and a sponge which are exactly the same size. The brick and the sponge will occupy the same amount of s

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问题     Suppose you have a brick and a sponge which are exactly the same size. The brick and the sponge will occupy the same amount of space, but they will not weigh the same. If two things are exactly the same size, but differ in weight, then they have different densities. The one with the greater weight for the same size has the greater density. The one with lower weight for the same size has the lower density.
    Now apply this idea to fluids. You know that water and air expand when they are heated and therefore become less dense. If you poured a cupful of very hot water and let it cool, you would have a little less than a cupful of cool water. If you weighed a cupful of very hot water and a cupful of cool water, the cupful of very hot water would weigh a little less.
    The cupful of cool water, being a little heavier than the cupful of hot water, would really have a little more water in it. Cold water is denser than hot water. Likewise, cold air is denser than hot air. The greater weight of the denser material makes it fall to the bottom of the container. It pushes up the less dense material.
Two things are the same size but differently weigh. That means______.
选项 A、the heavier one has lower density
B、the lighter one has greater density
C、they have the same density
D、they have different density
答案D
解析 第一段。选项A、B、C都与文中所说相反。重量大则密度大，同样大小但未必同样的重量，不同的重量则密度不同。选项D是正确的。
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