题目内容：
Questionsarebasedonthefollowingpassage:
Treesareusefultomaninthreeimportantways:theyprovidehimwithwoodandotherproducts;theygivehimshade;andtheyhelptopreventdroughtsandfloods．
Unfortunately,inmanypartsoftheworld,manhasnotrealizedthatthethirdoftheseservicesisthemostimportant．Inhiseagernesstomakemoneyfromtrees,hehascutthemdowninlargenumbers,onlytofindthatwithoutthemhehaslostthebestfriendshehad．Andbesides,heisusuallytoocalesstoplantandlookafternewtrees．Sotheforestsslowlydisappear．
Thisdoesnotonlymeanthatmanwillhavefewertrees．Theresultsareevenmoreserious:forwheretherearetrees,theirrootsbreakupsoil--allowingtheraintosinkin--andalsobindthesoil,thuspreventingitfrombeingwashedawayeasily;butwheretherearenotrees,theranfallsonhardgroundandflowsaway,causingfloodsandcantingawaytherichtop-soil．Whenallthetop-soilisgone,nothingremainsbutworthlessdesert．
Twothousandyearsagoarichandpowerfulcountrycutdownitstreestobuildwarships,withwhichtogainitselfanempire．Itsetuptheempirebut,withoutitstrees,itssoilbecamepooranditgrewweak．
Whentheempirefelltopieces,thehomecountryfounditselffacedwithfloodsandstarvation．
Whatisthemostimportantserviceoftreestomanaccordingtothepassage? A．Theyhelphimmakemoney
B．Theygivehimwoodandotherproducts
C．Theyhelphimpreventdroughtsandfloods
D．Theygivehimshade
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答案解析：