Researchers agree that a future mission (任务) should bring back physical samples (样品). But some scientists worry about the risk that this could introduce dangerous foreign creatures to Earth. “The problem here is how to get the samples back,” Squyres replied. “I think it is our responsibility to limit any risk.”

　　52. ________ prove that water existed on Mars.

　　A. The physical samples with salty chemicals

　　B. The marks on rocks and salty chemicals

　　C. Scientific achievements of 2004

　　D. Creatures from Mars

　　53. Which of the following is TURE according to the text?

　　A. It is now certain that there was water on Mars.

　　B. The water on Mars was like an ocean.

　　C. Evidence of life on Mars has been found.

　　D. Search for life on Mars is a great risk to humans.

　　54. How did scientists draw the conclusion that there was water on Mars?

　　A. The world’s leading magazine has announced the fact.

　　B. Scientists have taken many pictures and chemical readings on the Mars.

　　C. Astronauts have got some rocks from Mars.

　　D. Scientists have made a study of the pictures and readings sent back by Mars detector.

　　55. It can be inferred that, if the creatures from Mars came to Earth, ________.

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