Comets(彗星) are parts of our solar system. Like the planets, they go around the sun.

But comets are not made of solid rocks like planets. A comet is a ball of dust, stones, and ice. Many people call comets dirty snowballs.

The “snowball” may be only a few miles across. But when the sun heats the “snowball,” much of it is changed to gases. The gases spread out and form the comet’s head, which may be thousands of miles across.

A comet moves fast in its trip around the sun. But when we see it, the comet does not seem to move. That’s because it is so far away. The moon moves fast, but when you look at it, you can’t see any motion(运动). That’s because the moon is far away.

Each year astronomers(天文学家) discover new comets. Some of them are seen only once. They make one trip around the sun and then go away out into space. The sun’s gravity cannot hold them.

Other comets, like Halley, keep returning. They have been captured by the sun. Halley’s earliest visit was probably 3,000 years ago. It may keep returning for another 3,000 years.

But every time a comet goes around the sun, the comet loses part of itself. Gases and dust are pulled out of the comet. That’s why Halley is now dimmer（暗淡的） than it used to be. Next time it visits us, in 2062, it may be even dimmer. Each visit it may get dimmer and dimmer, until it finally disappears.

1.What is this passage mostly about?

2.How does the writer help the reader understand what a comet looks like?

3.What is the most likely reason the writer wrote this passage?

If you look at the sky one night and see something moving and shining that you have never seen before, it might be a comet (彗星).

　　A comet sometimes looks like a star. Like a planet, a comet has no light of its own. It shines from the sunlight it reflects (反射). Like the earth, a comet goes round the sun, but on a much longer path (轨道) than the earth travels.

       If a comet isn’t a star, what is it then?

　   Some scientists think that a large part of a comet is water frozen into pieces of ice and mixed with iron and rock dust and perhaps a few big pieces of rock. When sunshine melts (融化) the ice in the comet, great clouds of gas go trailing after it. These clouds, together with the dust, form a long tail.Many people perhaps have seen a comet. However no one knows how many comets there are. There may be millions of comets, but only a few come close enough for us to see.

　　An Englishman named Edmund Halley, who lived from 1656 to 1742, found out a lot about the paths that comets take through the sky. Some comets move out of our sight and never come back. Others keep coming back at regular times. A big comet that keeps coming back was named after Halley because he was the one who worked out when it would come back again. Maybe you have ever seen Halley’s Comets because the last time it came close to the sun and the earth was in the year 1986. Then people all over the world were outside at night to look at it. You will probably be able to see Halley’s Comets when it comes near the earth again.

1.A comet is like ________.

A. sun      B. moon     C. sunlight    D. the earth

2.A large part of a comet is ______.

A. water and rock

B. water frozen into pieces of ice and mixed with iron

C. ice, iron and rock dust

D. only a few big pieces of rock

3.Maybe many people _______.

A. haven’t seen any comets      B. have seen all comets

C. have seen a comet at daytime  D. have seen a comet

4.Some comets keep coming back ________.

A. at any time  B. at noon  C. at regular times  D. at daytime

5.Halley’s Comets came back _____.

A. in 1990    B. in 1980     C. in 1986      D. in 1989

阅读下面短文，把方框中的句子还原到文中，然后完成第五小题。（每题1分，共5 分）

If you look at the sky one night and see something moving and shining that you have never seen before, it might be a comet (彗星).

　　A comet sometimes looks like a star.? ____1.____ It shines from the sunlight it reflects (反射). Like the earth, a comet goes round the sun, but on a much longer path (轨道) than the earth travels.

??? If a comet isn’t a star, what is it then?

　? Some scientists think that a large part of a comet is water frozen into pieces of ice and mixed with iron and rock dust and perhaps a few big pieces of rock. ____2.______ These clouds, together with the dust, form a long tail.

Many people perhaps have seen a comet. _____3._____ .There may be millions of comets, but only a few come close enough for us to see.

An Englishman named Edmund Halley, who lived from 1656 to 1742, found out a lot about the paths that comets take through the sky.? ______4.____? . Others keep coming back at regular times. A big comet that keeps coming back was named after Halley because he was the one who worked out when it would come back again. Maybe you have ever seen Halley’s Comets because the last time it came close to the sun and the earth was in the year 1986. Then people all over the world were outside at night to look at it. You will probably be able to see Halley’s Comets when it comes near the earth again.

A. However no one knows how many comets there are.

B. Some comets move out of our sight and never come

C. Like a planet, a comet has no light of its own.

D. When sunshine melts (融化) the ice in the comet, great clouds of gas go trailing after it.

5.Which of the following is not true?

A. A large part of a comet is ice, iron and rock dust

B. Some comets keep coming back at any time

C. Halley’s Comets came back in 1986

D. Maybe many people have seen a comet

Comets(彗星) are parts of our solar system. Like the planets, they go around the sun.
But comets are not made of solid rocks like planets. A comet is a ball of dust, stones, and ice. Many people call comets dirty snowballs.
The “snowball” may be only a few miles across. But when the sun heats the “snowball,” much of it is changed to gases. The gases spread out and form the comet’s head, which may be thousands of miles across.
A comet moves fast in its trip around the sun. But when we see it, the comet does not seem to move. That’s because it is so far away. The moon moves fast, but when you look at it, you can’t see any motion(运动). That’s because the moon is far away.
Each year astronomers(天文学家) discover new comets. Some of them are seen only once. They make one trip around the sun and then go away out into space. The sun’s gravity cannot hold them.
Other comets, like Halley, keep returning. They have been captured by the sun. Halley’s earliest visit was probably 3,000 years ago. It may keep returning for another 3,000 years.
But every time a comet goes around the sun, the comet loses part of itself. Gases and dust are pulled out of the comet. That’s why Halley is now dimmer（暗淡的） than it used to be. Next time it visits us, in 2062, it may be even dimmer. Each visit it may get dimmer and dimmer, until it finally disappears.
【小题1】What is this passage mostly about?