六、阅读理解。
①How the Grand Canyon (大峡谷) was created remains one of the greatest geological (地质学的) mysteries. Some geologists believed that the process was a slow one: The Colorado River which goes through the canyon slowly cut deeper and deeper into the ground over millions of years. However, some rocks of the volcanoes (火山群), which have been discovered in the canyon recently, suggest that the canyon was down-cut instead.
②Down-cutting is when a lot of water rushes over a place with enough force to cut deeply into the ground and leaves behind a canyon. Such a flood (洪水) usually happens when a natural or man-made dam (大坝) breaks.
③Robert Webb, a research geologist, 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 (火山岩浆) from the nearby volcanoes flooded into the river. The lava became 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, forming a flash flood that pushed the down-cutting process and deepened the canyon.
④Down-cutting is not only an earthly event. Photos sent back from Mars suggest that the process has happened there, too, according to many other scientists. The photos, taken by the Mars Global Surveyor, show that a huge lake appeared on Mars 3.5 billion years ago. The lake flooded into a large nearby hole. On one side of the hole, a flash flood quickly shaped a huge canyon.
()41. In the past, the Grand Canyon was believed to be made by.
A. a river running slowly over years B. a volcano breaking suddenly
C. the rock rising from the earth D. the ground moving slowly
()42. Which picture shows the down-cutting of the canyon?

()43. The photos of Mars are mentioned in the last paragraph to.
A. introduce a topic B. give an example
C. show new ideas D. compare with others
()44. The RIGHT structure of the passage may be.

41－44.ACBA