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Treesareusefultomaninthreeimportantways:theyprovidehimwithwoodandotherprod-ucts;theygivehimshade;andtheyhelptopreventdroughtsandfloods.
Unfortunately,inmanypartsoftheworld,manhasnotrealizedthatthethirdoftheseservicesisthemostimportant.(78)Inhiseagernesstomakemoneyfromtrees,hehascutthemdowninlargenumbers,onlytofindthatwithout,themhehaslostthebestfriendshehad.Andbesides,heisusuallytoocarelesstoplantandlookafternewtrees.Sotheforestsslowlydisappear.
Thisdoesnotonlymeanthatmanwillhavefewertrees.Theresultsareevenmoreserious:forwheretherearetrees,theirrootsbreakupsoil--allowingtheraintosinkin--andalsobindthesoil,thuspreventingitfrombeingwashedawayeasily;butwheretherearenotrees,therainfallsonhardgroundandflowsaway,causingfloodsandcarryingawaytherichtop-soil.Whenallthetop-soilisgone,nothingremainsbutworthlessdesert.
Twothousandyearsagoarichandpowerfulcountrycutdownitstreestobuildwarships,withwhichtogainitselfanempire.Itsetuptheempirebut,withoutitstrees,itssoilbecamepooranditgrewweak.Whentheempirefelltopieces,thehomecountryfounditselffacedwithfloodsandstarvation.
Whatisthemostimportantserviceoftreestomanaccordingtothepassage？查看材料
A.Theyhelphimmakemoney
B.Theygivehimwoodandotherproducts
C.Theyhelphimpreventdroughtsandfloods
D.Theygivehimshade
请帮忙给出正确答案和分析，谢谢！