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第3部分：概括大意与完成句子(第23--30题，每题1分，共8分)
　　下面的短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为第2～5段每段选择1个最佳标题；(2)第27～30题要求从所给的6个选项中为每个句子确定1个最佳选项。
Learn about Noble Gases（惰性气体）

　　1 Have you ever ridden on a balloon? Many tourist spots offer balloon rides in order for people to see the beauty of a place from above. A balloon contains a noble gas called hellum（氦）. Formerly, balloons contained hydrogen but hydrogen is very flammable and dangerous when uncontrolled. Therefore, people shifted to helium, which is safer. Helium is safe because it has the properties of the noble gases.
　　 2 People once belleved that noble gases couldn’t chemically react at all. For this reason, they were called inert gases（惰性气体）. They were also listed under Group 0 in the old periodic table because scientists believed that the gases have zero valence（价）electrons in their outer shell. This was later proven to be untrue when some noble gas compounds were discovered.
　　 3 The gases are elements, which share similar properties. These properties include being monoatomic, colorless, odorless, being able to conduct electricity, and having low chemical reactivity. Noble gases include Helium, Neon, Argon, Krypton, Xenon and Radon. These are all found in Group 18, in the rightmost column of the periodic table. If you look at the periodic table, you will notice that these elements are the only ones, which do not have a charge. Helium has the lowest molecular（分子的）weight while Radon is the heaviest.
　　 4 Remember that chemical reactions occur because atoms have valence electrons, which are electrons in their outer shell. When the outer shell is “unfilled” or the required number of electrons is not yet complete, the atom is more reactive. Noble gases have a full outer shell, meaning that they have complete electrons in their outer shell. This complete number varies. For instance, the outer shell of Helium has 2 valence electrons while the outer shell of Xenon has 8 electrons. Nowadays, there remains to be a few noble gases because of the low chemical reactivity of these said gases.
　　 5 because of their properties, noble gases have many important applications. They are widely used in medicine and industries. For instance, liquid Helium is used for superconducting magnets（磁体）. These magnets are very important in physics and medicine. When a doctor suspects that a person’s brain has been damaged, he might request for Magnetic Resonance Imaging(MRI). MRI allows the doctor to “see” the brain, without operating on the patient.
　　 23. paragraph 2________
　　 24. paragraph 3________
　　 25. paragraph 4________
　　 26. paragraph 5________
　　 A. How were noble gases discovered?
　　 B. what is the periodic table?
　　 C. what are the applications of noble gases?
　　 D. How were noble gases understood in the past?
　　 E.What causes the low chemical reactivity of noble gases?
　　 F. What are noble gases?
　　 27. Noble gases are not very chemically .
　　 28. Among the elements of noble gases Helium is the .
　　 29. The required number of electrons in noble gases’outer shell is .
　　 30. MRI may make operating on the patient .
　　 A. complete
　　 B. reactive
　　 C.unnecessary
　　 D. flammable
　　 E. important
　　 F. lightest