A comet seems to be composed of gas, dust and ice. There is a good deal of solid, frozen material. As the mass approaches the Sun, the outer frozen parts begin to melt, and more gases form. The pressure of the Sun’s radiation drives the dust and gases outwards in a long, streaming “tail”, perhaps several tails. For this reason the tail always points away from the Sun. And as the comet moves farther from the radiation, the tail gets smaller and at last disappears.

(85 words)

　　What is the passage mainly about?
　　 A) Comet and the Sun.　　　
　　 B) Comet and its tails.　　　　
　　 C) The movement of comets.　　　　
　　 D) The radiation of the Sun.

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