题目内容：
根据下面内容，回答题 SupposewebuiltarobottoexploretheplanetMars(火星).Weprovidetherobotwithseeingdetectorstokeepitawayfromdanger.Itispoweredentirelybythesun.Shouldweprogramtherobottobeequallyactiveatalltimes?No.Therobotwouldbeusingupenergyatatimewhenitwasnotreceivingany.Sowewouldprobablyprogramittoceaseitsactivityatnightandtowakeupatdawnthenextmorning.
Accordingtotheevolutionarytheoryofsleep,evolutionequippeduswitharegularpatternofsleepingandwakingforthesamereason.Thetheorydoesnotdenythatsleepprovidessomeimportantrestorative(促进康复的)functions.Itmerelysaysthatevolutionhasprogrammedustoperformthosefunctionsatatimewhenactivitywouldbeinefficientandpossiblydangerous.However,sleepprotectsusonlyfromthesortoftroublewemightwalkinto;itdoesnotprotectusfromtroublethatcomeslookingforus.Sowesleepwellwhenweareinafamiliar,safeplace,butwesleeplightly,ifatall,whenwefearthatbearswillnoseintothetent.
Theevolutionarytheoryaccountswellfordifferencesinsleepamongcreatures.Whydocats,forinstance,sleepsomuch,whilehorsessleepsolittle?Surelycatsdonotneedfivetimesasmuchrepairandrestorationashorsesdo.Butcatscanaffordtohavelongperiodsofinactivitybecausetheyspendlittletimeeatingandareunlikelytobeattackedwhiletheysleep.Horsesmustspendalmostalltheirwakinghourseating,becausewhattheyeatisverylowinenergyvalue.Moreover,theycannotaffordtosleeptoolongortoodeeply,becausetheirsurvivaldependsontheirabilitytorunawayfromattackers.
Theauthorusestheexampleoftherobotinspaceexplorationtotellus__________. A．thedifferencebetweenrobotsandmen
B．thereasonwhymenneedtosleep
C．abouttheneedforrobotstosavepower
D．aboutthedangerofmenworkingatnight
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答案解析：