题目内容：
Questionsarebasedonthefollowingpassage．
Virusesarenasty.TheverywordstemsfromLatinfor"poison".Aclassofthemcalledretroviruses(反转灵病毒)isparticularlydangerousandsubtle.These,whichincludeAIDS-causingHIV,injecttheirgeneticmaterialintoahost'sDNA,whereitisreplicatedalongwiththehealthygenes.Theycanaffectwhichgenesare"expressed",withpotentiallyprofoundconsequencesforhealth.Humanretrovirusesunderstandablygetmostattention,buttheythreatenotheranimals,too.Onehasrecentlybeenwreakinghavocinthemarsupial(有袋动物的)world.Insomeplaces,ithaswipedout80%ofthekoalapopulation.
Whenaretrovirusinfectscellsinvolvedinreproduction,bitsofitsgeneticcodecangetpasseddowntothehost'soffspring.Theseremainingbitsarecalledendogenousretroviruses(ERVs)(内生反转录病毒).Koalaretrovirusisintheprocessofbecomingone.Generationslater,whatwasonceharmfulbecomesjustpartofthespecies'genome(染色题).Butrightnow,itiscausingimmune-systemproblemsandcancers.
Retrovirusescanalsopassbetweendifferentspecies(asHIVjumpedfromprimatestohumans).SimilaritiesintheERVsfoundindifferentanimalsmightshedlightonhowchainsoftransmissionsoccurred.Inapaper,AlexHaywardandPatricJemofUppsalaUniversityinSwedencomparedthegeneticmotifs(图谱)inthousandsofknownERVsin60species.Theylookedinparticularataclasscalled"gamma-ERVs"--atypecommonlyfoundinmammalsandtowhichkoalaretrovirusbelongs.
Thepatternstheyuncoveredsuggestedthatonefactoraffectingtransmissionmightbediet:nogamma-ERVswereseeninthegenome(基因组)ofsixlargevegetarianmammalsstudied,butcamivoreshadanabundanceofthem.Geographyandecologymightplayarole,too,asmoregamma-ERVswerefoundinanimalsdwellingclosetotheground.CombinethesetwofactorsanditislittlewonderthatmorethanhalfofallERVswerefoundinrats.Geneticpatternssuggestthatinfectionsjumpingbetweenrodents(啮齿动物)andotheranimals(mostnotablyprimates)arecommonplace.Ratsareinfamousforpassingdiseases.Asitturnsout,theyarerepositoriesforretroviruses,too,ontopofmoreordinarygerms.
Theresearcherswouldnowliketousetheircomparativemappingtechniquetolookatthegenomeofblack"ship"rats(notincludedinthisstudy).Thismightyieldpatternsshowingacloserrelationshipwithkoalaretrovirus.Ifso,theyimagineascenarioofinfectedratshitchingtheirwayaboardshipstoAustralia,andspreadingthevirustokoalasthroughanothervector(带菌者),suchasabat.Thisisspeculative;theexactmodeofinter-speciesretroviraltransmissionisstillunknown.Butthesefindingsprovideprovocativenewavenuesofenquiry--andperhapsanotherreasontodislikerats.
Whatcanweknowaboutretrovirusesaccordingtothefirstparagraph? A．Retrovirusesaredifficulttogetinfected.
B．HIVdoesn'tbelongtoretroviruses.
C．Animalretrovirusesgetmostattention.
D．Aretrovirushasbroughtadisastertomarsupialanimals.
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