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?
A．Comets and the sun.
B．Why comets are like snowballs.
C．Who discovered the first comet.
D．Facts about comets.小题2:How does the writer help the reader understand what a comet looks like?
A．By telling about a famous one.
B．By explaining how far away it is.
C．By describing how fast it moves.
D．By describing it as a dirty snowball.小题3:What is the most likely reason the writer wrote this passage?
A．To give information about comets.
B．To tell readers about famous comets.
C．To explain the importance of comets.
D．To encourage readers to look for comets.
和
互为同位素，物理性质不同，化学性质几乎完全相同