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閱讀理解：閱讀下面的短文，然后根據(jù)短文內(nèi)容選擇最佳答案。
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(運(yùn)動)．That's because the moon is far away．
Each year astronomers(天文學(xué)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．
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