Question

Read the following three passages．Each passage is followed by several questions or unfinished statements．For each of them there are four choices marked A, B, C and D．Choose the one that fits best according to the information given in the passage you have just read． 　　How the Grand Canyon was created remains one of the geology’s greatest mysteries．Some evidence suggests that the process was a gradual one in which the Colorado River(which runs through the canyon)slowly cut deeper and deeper into the ground over millions of years．But volcanic(火山的)rock samples taken from the canyon(峽谷)now suggest that the canyon was downcut instead． 　　Downcutting is when a flood of water rushes over a landscape with enough force to cut deeply into the ground and leaves behind a canyon．Such a flood is usually released when a natural or artificial dam bursts． 　　Robert Webb, a research geologist with U．S．Geological Survey, says natural dams seem to have formed and broken across the Colorado River several times during the last million years．The dams were built when lava(熔巖)from the eruptions(爆發(fā))of nearby volcanoes flowed into the river．The lava hardened into hard rocks and blocked the river, causing it to back up and form a lake．Each time the lake grew so huge that it broke the rock dam, releasing a flash flood that furthered the downcutting process and deepened the canyon．The most recent dam break happened 165,000 years ago, says Webb, and released 15 million cubic(立方)feet of water per second． 　　Downcutting is not just an earthly event．Satellite photos sent back from Mars(a planet)suggest that the process has happened there, too, say researchers at the National Air and Space Museum． 　　The photos, taken by the Mars Global Surveyor, indicate that an enormous lake existed on Mars 3.5 billion years ago．The lake, which was big enough to flood Texas and California, eventually spilled into a large nearby hole．One edge of the hole broke, releasing a flash flood that quickly carved out a grand canyon about 2,100 meters deep and 885 kilometers long．Astronauts call the canyon MA’adim Vallis． 　　The existence of downcutting on Mars is just one more piece of evidence that the cold, dry planet was once warm and wet． (1) In the past, deep canyons were believed to have formed ________． [　　] A． as a result of a sudden break of volcanoes B． due to river flows over millions of years C． owing to the collapse of artificial dams D． thanks to our ancestors’ creative work (2) From the passage we can guess that ________． [　　] A． several volcanoes broke out in the Colorado River B． several great lakes existed on the upper parts of the Colorado River C． people built high dams on the upper parts of the Colorado River D． the Colorado River crossed the Grand Canyon millions of years ago (3) From the last three paragraphs we learn that ________． [　　] A． there are great lakes on Mars B． there are active volcanoes on Mars C． there might be cities on Mars D． there might be life on Mars (4) The passage can be titled as ________． [　　] A． Lakes on Earth and Mars B． The Cause of Lake Formation C． The Force of Dam Breaking D． Canyons－Results of Flood Cutting

Answer 1

答案：1．B;2．B;3．D;4．D;