阅读理解 People once widely believed that intelligent life existed on Mars. The 19th-century 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 20th century 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 or 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.

单选题 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.

【正确答案】 C

【答案解析】本题的线索在文章的第一段。由“The fines were thought to have been a system of canals that had been built to irrigate the surface.”可知，只有C项可以使人们相信火星上存在着智能生命，其他项都不是本题的依据。

单选题 The "canals" on Mars have proved to be ______according to the passage.

【正确答案】 B

【答案解析】本题的线索在文章的第一段。由“It is now clear that the“canals”—perhaps the most spectacular geologic features of Mars—are natural valleys where ancient rivers once flowed.”可知，火星上的河渠实际上是远古时河流流过时形成的天然峡谷。因此本题的正确答案为B，其他项都是干扰项。

单选题 What are the Mars' seasonal changes in color believed to be?

【正确答案】 C

【答案解析】本题的线索在文章的第二段。由“Once attributed to the rapid spread of some life-form，”可知颜色变化是由于一些生命形式的迅速扩散。只有C项才是文中所提及的原因。

单选题 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.

【正确答案】 A

【答案解析】本题的线索在文章的第三段。由“In 1996 scientists discovered organic compounds and minerals in a meteorite(陨石)，consisting of Martian rock，”可知，火星陨石里含有有机体混合物和矿物质，这可以证明火星上存在生物体的可能性。因此本题的正确答案是A项。

单选题 How many arguments in this passage lead to the belief of the existence of life on Mars?

【正确答案】 B

【答案解析】通过分析全文可知，本题的论据包含在三个段落中，如“a system of canals” “the rapid spread of some life-form”“organic compounds and minerals in a meteorite(陨石)”，因此，本题的正确答案为B。