题目内容：

    Clouds are made of small drops of water or tiny pieces of ice. The water and ice in clouds come from the Earth ’s surface，especially from lakes，rivers，and oceans. Water on the surface is heated. It changes into a gas called water vapor(水蒸气). Water vapor goes up into the air.

The higher up you go，the colder the air gets. When water vapor gets cold，it changes back into drops of water. If it gets cold enough，the water changes to ice. When the drops of water and bits of ice are so small，you see the water and ice in the sky as clouds. Scientists who study weather divide clouds into three families. Three families are called high clouds，middle clouds，and low clouds.

High clouds are made of bits of ice. High clouds can be 5 miles or more above the Earth’s surface. Rain and snow do not fall from these clouds.

Middle clouds are made of drops of water. They form about 2 to 4 miles up in the sky. You can see through them，but you can’ t see the Sun and the Moon clearly because of them.

Low clouds are made of drops of water. Most low clouds are less than a mile high. Rain or snow usually falls from these clouds.

1.Where do the water and ice in clouds come from?

The water and ice in clouds come from____________.

2.When does the water vapor change back into drops of water?

When water vapor____________，it changes back into drops of water.

3.Why can’ t you see the sun and the moon clearly?

You can’t see the sun and the moon clearly because of _____________.

4.What does “It” refer to in the first paragraph?

It refers to______________.

5.Where does the rain or snow usually fall from?

Rain or snow usually falls from________________.