Down-cutting is not just an earthly event. Satellite photos sent back from Mars suggest that the process has happened there, too, say many other researchers.

　　The photos, taken by the Mars Global Surveyor, indicate that an enormous lake existed on Mars 3.5 billion years ago. The lake spilled into a large nearby hole. One edge of the hole broke, releasing a flash flood that quickly carved out a grand canyon.

　　The existence of down-cutting on Mars is just one more piece of evidence that the cold, dry planet was once warm and wet.

　　24. 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 burst of artificial dams

　　D. thanks to our ancestors’ creative work

　　25. According to the passage, which of the following statements is TURE?

　　A. Several volcanoes broke out on Mars directly creating grand canyons.

　　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 to form down-cutting.

　　26. From the last three paragraphs we learn that _______.

　　A. there are great lakes on Mars

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