完型填空：

　　Scientists no w believe the earth’s magnetic poles(地磁极)have changed many times．One 　　1　　 change may have taken place about 330,000 BC．This 　　2　　 is about when modem man first 　　3　　 on the earth．

　　Many 　　4　　 now believe the magnetism(磁力现象)of the earth may have a 　　5　　 influence(影响)on life itself．In the 　　6　　 past many kinds of animals and plants died out suddenly, 　　7　　 the North and South magnetic poles changed 　　8　　．

　　Animals, birds and 　　9　　 people are to some degree controlled by magnetic forces When there is a 　　10　　 increase in magnetic strength; many animals have 　　11　　 in finding their way．Mice, 　　12　　 near a strong magnet, lose their hair and die 　　13　　．And when the earth's magnetism suddenly decreases, the 　　14　　 of men or women who kill themselves increases．

　　How 　　15　　 does the earth’s magnetism 　　16　　 our lives?We cannot know for 　　17　　．We do know this magnetism is getting weaker and we know that in 2,500 years the magnetic poles may change position 　　18　　．And we know when 　　19　　 happened in the past there were enormous 　　20　　 in life on Earth．

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