根据文章,回答 67~70 问题DHave you ever noticed the colour of the water in a river or strea

根据文章，回答67～70问题D

Haveyouevernoticedthecolourofthewaterinariverorstr eamafteraheavyrainfall?Whatd oyouthinkcausedthischangeincolour?Itissoilthathasbeenwashedintotheriverfromtheriverbankorfromt}lenearbyfields．

ComponentsofSoft

Soilismadeupofanumberoflayers(层)，eachhavingitsowndistinctivecolourandtexture．Theupperlayerisknownasthelitter．Itactslikeablanket．limitingtemperaturechangesandreducingwaterloss．Thetopsoillayerismadeupofsmallparticlesofrockmixedwithrottenplantandanimalmattercalledhumus(腐殖质)，whichisblackandgivesthetopsoilitsdarkcolour．Thislayerisusuallyrichinnutr ients，oxygen，andwater．Belowthetopsoilisthesubsoil，alayerthatcontainsmorestonesmixedwithonlysmallamountsoforganicmatter．Thislayerislighterincolourbecauseofthelackofhumus．Beneaththesoilliesalayerofbedrock．

Soilformsfromthebottomup．Overtimebedrockisattackedbyrain,wind，frost,andsnow．Itisgraduallybrokendownintosmallerparticlesinaprocesscalledweathering．Plantsbegintogrow，androttenmaterialsenrichthetopsoil．MostofthesoilinEasternCanada．forexample．Wasformedfromweatheredrockthatwasexposedwhentheicedisappearedl2．000yearsag0．

WaterBeneaththeSoil

Surfacewatercollectsandflowsabovethegroundinlakes．ponds．andrivers．onceinthesoilorrock，itiscalledgroundwater．Gravitypullsgroundwaterthroughthesoilinaprocesscalledpercolation(渗透)．Eventuallythewaterreachesalayercalledthewatertable ．Underthisisbedrockthroughwhichwatercannotpercolate．

Aswaterpercolatesdownward，itd issolvesorganicmatterandmineralsfromthesoilandcarriesthemtodeeperlayers．Thiscausesaseriousproblembecauseplantsrequirethesenutr ientsforgrowth.

SoilpH

Soilcanbeacidic．neutr al．orbasic．ThepHofthesoilisdeterminedbythenatureoftherock．

fromwhichitwasformed．andbythenatureoft}leplantsthatgrowandrotinit．

TheacidityofrainandsnowcanlowerthepHofthegroundwaterthatentersthesoil．Byburningfossilfuelssuchascoal，oilandgasoline，humanshavebeencontr ibutingtohigherlevelsofacidityinmanysoils．Whenfossilfuelsareburned．gasesarereleasedintotheairandthenfallbacktoearthasacidrain．Acidsoilincreases出eproblemofcarryingnutr ientstolowersoillevels．Asnutr ientsareremoved，soilislessfertile．Plantsgrowmoreslowlyinacidicsoil，andalsobecomeeasilyattackedbydiseases．

第12题：Thelayerofsoilthatprovidesnecessarynutr ientsforplantgrowthiscalled．

A．1itter

B．topsoil

C．humus

D．subsoil