Winterweatherhasreturnedtonorthernareasoftheworld.InmuchoftheUnitedStateswintermeansthereturnofsnow.

Snowisaformoffrozenwater.Itcontainsmanygroupsoftinyiceparticles(颗粒)calledsnowcrystals(晶体).Thesecrystalsgrowfromwaterparticlesincoldclouds.Theyusuallygrowaroundapieceofdust.Allsnowcrystalshavesixsidesbuttheygrowindifferentshapes.Theshapedependsmainlyonthetemperatureandwaterlevelsintheair.Theshapeofasnowcrystalmaychangefromoneformtoanotherasthecrystalpassesthroughlevelsofairwithdifferenttemperatures.Snowcontainsmuchlesswaterthanrain.Aboutseventy-fivecentimetersof snowhasasmuchwaterastwoandone-halfcentimetersofrain.

Generallythecolorofsnowandiceappearswhite.Thisisbecausethelightweseefromthesuniswhite.Mostnaturalmaterialstakeinsomesunlight.Howeverwhenlighttravelsfromairtosnowsomelightissentbackorreflected(反射).Snowcrystalshavemanysurfacestoreflectsunlight.Itisthislightthatgivessnowitswhitecolor.

Muchofthewaterweusecomesfromsnow.Meltingsnowgiveswatertoriversandcrops.Snowfallhelpstoprotectplantsandsomewildanimalsfromcold.Freshsnowismadelargelyofairwhichiskeptamongthesnowcrystals.Becausetheairhastroublemovingthemovementofheatisgreatlyreduced.HoweversnowisresponsibleforthedeathsofhundredsofpeopleintheUnitedStateseveryyear.Manypeopledieintrafficaccidentsonroadsthatarecoveredwithsnoworice.

66.Inthecenterofasnowcrystalthereis______.

A.air B.water C.dust D.sunlight

67.Iffifteencentimetersofsnowcompletelymeltstheheightofwaterwillbe______.

A.0.5centimeter B.1centimeter C.1.5centimeters D.15centimeters

68.Accordingtothetextsnowlookswhitebecause_______.

A.snowisactuallywhite B.theiceparticlesarewhite

C.thereflectionoflightmakesitlookwhite D.thereisalotofairinthesnow

69.Howcansnowprotectplantsfromthecold?

A.Snowgivesthemmuchheat.

B.Snowturnsintowaterandwatersthem.

C.Snowmakesthemkeepmoving.

D.Snowcontainsairwhichhastroublemoving.