托福阅读真题解析：火星上的流水

southern highlands into the northern plains. The onrushing water arising from these flashfloods likely also formed the odd teardrop-shaped “islands” (resembling the miniature versionsseen in the wet sand of our beaches at low tide) that have been found on the plains close tothe ends of the outflow channels. Judging from the width and depth of the channels, the flowrates must have been truly enormous―perhaps as much as a hundred times greater than the105 tons per second carried by the great Amazon river. Flooding shaped the outflow channelsapproximately 3 billion years ago, about the same times as the northern volcanic plainsformed.

　　12. Look at the four squares [] that indicate where the following sentence could be added tothe passage.

　　These landscape features differ from runoff channels in a number of ways.

　　Where would the sentence best fit?

　　13. Directions: An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the mostimportant ideas in the passage. Some sentences do not belong in the summary because theyexpress that are not presented in the passage or are minor ideas in the passage. This questionis worth 2 points.

　　There is much debate concerning whether Mars once had water.

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　　Answer choices

　　1. Mars’ runoff and outflow channels are large-scale, distinctive features that suggest thatlarge quantities of liquid water once flowed on Mars.

　　2. Although some researchers claim that Mars may once have had oceans, others dispute this,pointing to an absence of evidence or offering alternative interpretations of evidence.

　　3. Various types of images have been used to demonstrate that most of Martian surfacecontains evidence of flowing water.

　　4. The runoff and outflow channels of Mars apparently carried a higher volume of water andformed more extensive networks than do Earth’s river systems.