Questions 21-25 are based on the following passage :
　　People once widely believed that intelligent life existed on Mars . The 19th-centruy discovery of what appeared to be geometric designs cut across the surface was taken as evidence. The lines were thought to have been a system of canals that had been built to irrigate the surface. It is now clear that the “canals” ---perhaps the most spectacular geologic features of Mars – are natural valleys where ancient rivers once flowed.
　　Another fragmented idea concerns the planet’s seasonal changes in color. Once attributed to the rapid spread of some life-form, these shifts are now known to develop from the movement of fine dust in the atmosphere.
　　By the close of the 20thcentury none of the many experiments conducted by spacecraft had ever found persuasive evidence of life. Nevertheless, speculation continued over the existence of some form of life ,in either the present of past. In 1996 scientists discovered organic compounds and minerals in a meteorite （陨石），consisting of Martian rock, that collided with Earth around 11,000 B.C. These compounds suggest that Mars may have been inhabited by organisms more than three billion years ago.
　　21. Why did people in the 19th century believe the existence of intelligent life on mars ?
　　A. Because the surface of Mars seemed to be geometric.
　　B. Because the lines were drawn across the surface of the planet.
　　C. Because a system of canals was thought to be there.
　　D. Because it was the Martians that built the canals.
　　22. The “canals” on Mars have proved to be according to the passage.
　　A. the minor geologic characteristics of Mars
　　B. natural valleys on the surface of Mars
　　C. rivers that have kept flowing since ancient times
　　D. a system that irrigates the whole surface
　　23.What are the Mars’ seasonal changes in color believed to be ?
　　A. The natural changes on the planet.
　　B. The seasonal cycles .
　　C. The movement of some life form .
　　D. The storm of dust in its atmosphere.
　　24. How is the 1996 discovery related to the possible existence of organisms on Mars?
　　A. The meteorite containing organic compounds is part of Mars.
　　B. A Martian rock struck Earth about 11,000 years ago.
　　C. The organisms came back to life after the collision with Earth.
　　D. The inhabiting organisms appeared more than three billion years ago.
　　25. How many arguments in this passage lead to the belief of the existence of life on Mars?
　　A. Tow
　　B. Three
　　C. Four
　　D. Five

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