Houston, Texas (June 8. 2002)–in 2004, the National Aeronautics and Space Administration

Houston, Texas (June 8. 2002)–in 2004, the National Aeronautics and Space Administration (NASA) will send two robots to separate places of Mars to seek out past or present signs of water. It is an exciting idea to send two robots driving over very different places of Mars at the same time, to be able to see what is on the other side of the hill.
Last month, NASA announced it was sending one robot to Mars, but after two weeks, it decided there was enough money for two. The robots will be sent up within two weeks of each other in May and June of 2003 from Kennedy Space Center. If all goes well, the two spacecraft will touch down on Mars, after a seven-and –a-half-month space flight, on January 2 and 20, 2004.
The robots, each weighing 150 kilograms, can cover 100 meters per day. They are designed to be able to examine the mineral content of the soil, and their special camera will take pictures of the lands and hills. Although they will be under control from the earth, the robots are able to move more freely compared to those sent up before them.
The actual landing points have not been determined yet, but the scientists say it will be in areas where they hope to find water.
49.According to the news report, scientists plan to send robots up to Mars to .
A.find out whether there is water on Mars B.see if robots can find minerals there
C.test how fast robots can drive there D.prove that robots can work on Mars
50.How long in between will the two robots be sent to Mars?
A.1 year B. 7.5 months C. 2 months D. 2 weeks
51. One of the important jobs for the robots on Mars is to .
A.study the soil B. walk everywhere
C.test the new camera D. find a suitable landing point
52. We can infer from the last sentence that scientists .
A. have changed the landing points many times B. hope to land the robots on the surface of water
C. are still working on the plan D. know where they can find water49—52: ADAC

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B解析解析：因为双缝干涉实验时，相邻条纹间距与光波波长成正比，所以a光波长比b光波长长，那么a光频率比b光频率小，a光在水中折射率比b光的折射率小，故a光在水中传播速度大于b光在水中传播速度．a光光子能量小于b光光子能量，故用b光照射时有可能发生光电效应．在原子跃迁时，从较高能级跃迁到较低能级时，级数相差越多，放出光子的能量越大，所以A、C、D错．

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