阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。Haveyouevernoticedthecolourofthewaterinariverorstreamafteraheavyrainfall?Whatdoyouthinkcausedthischangeincolour?Itissoilthathasbeenwashedintotheriverfromtheriverbankorfromthenearbyfields.ComponentsofSoilSoilismadeupofanumberoflayers(层),eachhavingitsowndistinctivecolourandtexture.Theupperlayerisknowasthelitter.Itactslikeablanket,limitingtemperaturechangesandreducingwaterloss.Thetopsoillayerismadeupofsmallparticlesofrockmixedwithrottenplantandanimalmattercalledhumus(腐殖质),whichisblackandgivesthetopsoilitsdarkcolour.Thislayerisusuallyrichinnutrients,oxygen,andwater.Belowthetopsoilisthesubsoil,alayerthatcontainsmorestonesmixedwithonlysmallamountsoforganicmatter.Thislayerislighterincolourbecauseofthelackofhumus.Beneaththesoilliesalayerofbedrock.Soilformsfromthebottomup.Overtimebedrockisattackedbyrain,wind,frost,andsnow.Itisgraduallybrokendownintosmallerparticlesinaprocesscalledweathering.Plantsbegintogrow,androttenmaterialsenrichthetopsoil.MostofthesoilinEasternCanada,forexample,wasformedfromweatheredrockthatwasexposedwhentheicedisappeared12,000yearsago.WaterBeneaththeSoilSurfacewatercollectsandflowsabovethegroundinlakes,ponds,andrivers.Onceinthesoilorrock,itiscalledgroundwater.Gravitypullsgroundwaterthroughthesoilinaprocesscalledpercolation(渗透).Eventuallythewaterreachesalayercalledthewatertable.Underthisisbedrockthroughwhichwatercannotpercolate.Aswaterpercolatesdownward,itdissolvesorganicmatterandmineralsfromthesoilandcarriesthemtodeeperlayers.Thiscausesaseriousproblembecauseplantsrequirethesenutrientsforgrowth.SoilpHSoilcanbeacidic,neutral,orbasic.ThepHofthesoilisdeterminedbythenatureoftherockfromwhichitwasformed,andbythenatureoftheplantsthatgrowandrotinit.TheacidityofrainandsnowcanlowerthepHofthegroundwaterthatentersthesoil.Byburningfossilfuelssuchascoal,oilandgasoline,humanshavebeencontributingtohigherlevelsofacidityinmanysoils.Whenfossilfuelsareburned,gasesarereleasedintotheairandthenfallbacktoearthasacidrain.Acidsoilinc