评定气体火灾危险性的主要指标为（）。
A.闪点
B.自燃点
C.爆炸极限
D.扩散性
E.化学活性
参考答案：B, C
解析：爆炸极限和自燃点是评定气体火灾危险性的主要指标。可燃气体的爆炸浓度极限范围越大，爆炸下限越低，越容易与空气或其他助燃气体形成爆炸性气体混合物，其火灾爆炸危险性越大。可燃气体的自燃点越低，遇有高温表面等热源引燃的可能性越大，火灾爆炸的危险性越大。因此，正确答案为B、C。
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| A young student was one day taking a walk with a professor, who was commonly 1 the students' friend. As they went 2 , they saw lying in the path a pair of old _3 , which were supposed to belong to a poor man who was working in a 4 close by. The student turned to the professor, saying, "Let's play the man a 5 : We will hide his shoes, and 6 ourselves behind those bushes, and wait to see his 7 when he cannot find them." "My young friend," answered the professor, "we should never 8 ourselves at the 9 of the poor. But you are 10 , and you may give yourself a much greater 11 by means of the rich man. Put a coin into each shoe, and then we will hide ourselves and 12 how the discovery affects him." The student did so, and they both placed themselves behind the bushes. The poor man soon finished his work, and came 13 the field to the path 14 he had left his coat and shoes. While 15 his coat he slipped his foot into one of his shoes; but feeling something 16 , he bent down to feel 17 it was, and found the coin. Astonishment and wonder were seen upon his face. He then looked around him on all sides, but no person was to be seen. He now put the money into his pocket, and continued to put on the other shoe; but his 18 was doubled on finding the other coin. He fell upon his knees, looked up to heaven and said aloud a fervent (热诚的) thanksgiving, in which he 19 his wife, sick and helpless, and his children without bread. The student stood there deeply 20 , and his eyes were filled with tears. | ||||
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