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xx年6月英语四级阅读理解及解析xx年6月英语四级阅读理解及解析 Merchant andpassenger shipsare generallyrequired tohave alife preserverfor everyperson aboardand inmany cases,a certainpercentage ofsmaller sizesfor children.Aording toUnited Statesrequirements,life preserversmust design,reversible capableof beingquickly adjustedto fitthe uninitiatedindividual,and mustbe sodesigned asto supportthe wearerin the water inan upright or slightly backward position. Sufficient buoyancy浮力to supportthe wearershould beretained bythe lifepreserver after48hours in the water,and itshould bereliable evenafter longperiod ofstorage.Thus itshould bemade ofmaterials resistantto sunlight,gasoline,and oils,and itshould benot easilyset onfire.The positionin whichthe lifepreserver willsupport aperson whojumps orfalls intothewateris mostimportant,as isits tendencyto turnthe wearerinthewater froma face-down positionto anuprightorslightlybackwardposition,...。