3.The best title for this passage would be       .

    A.How to Choose a Christmas Tree

    B.How Soldiers Spent Their Christmas

    C.The Perfect Christmas Tree

    D.The Christmas with an Ugly Christmas Tree

B

Scientists found that our Earth is not cold, hard rock all the way through. Not at all. Only the crust, the outside of the earth is cold, solid rock.

Below this cold solid crust of the Earth, there is hot rock that melts in places. Every now and then some of the hot, melted rock is pushed out.

This hot, melted rock expands, and can push its way out through a weak spot in the Earth’s crust. When this happens, a volcano is made.

The melted rock below the Earth’s crust can flow, like toothpaste (牙膏), if it is squeezed. If you squeeze a partly filled tube of toothpaste in one place, what happens?

The toothpaste flows to another part of the tube. If you press down on one part of the tube with a finger, what happens? This tube rises in another place, as the toothpaste flows there, making a hill.

Strangely enough, when a volcano forms, the Earth may be behaving like a tube of toothpaste. If the Earth is pressed down in one place, it may rise in another place. Hot, melted rock within the Earth may flow to another place as the Earth is pressed down. When this hot, melted rock breaks through the Earth’s crust, a volcano forms.

A volcano is only one kind of mountain. Other kinds of mountains form when the Earth’s crust is raised. But each time the Earth is raised in one place, something is pressing against the Earth in another place.

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