单项选择题

Passage 1
Questions 11 and 12 are based on the passage you have just heard.
[听力原文]11-12
Atoms are all around us. They are the bricks of which everything is made. Many millions of atoms are contained in just one grain of salt, but despite their small size they are very important. The way an everyday object behaves depends on what kinds of atoms are in it and how they act.
For instance, you know that most solid objects melt if they get hot enough. Why is this It is the effect of the heat on the objects’ atoms. All atoms move constantly. When they are hot, they move faster.
Usually the atoms ill an object hold together and give the object its shape. Bat if the object grows hot, its atoms move so fast that they break the force that usually holds them together. They move out of their usual places so that the object loses its shape. Then we say that the object is melting.

Why do most solid objects melt if they get hot enough（）

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