题目内容：
Questionsarebasedonthefollowingpassage.
Whatmakescellsage?Wearandtear,yes.Butbiologically,saysDr.DavidSinclair,professorofgeneticsatHarvardMedicalSchool,it'slackofoxygenthatsignalscellsthatit'stheirtimetogo.Withoutoxygen,theenergyenginesknownasthemitochondria(线粒体)becomelessefficientatturningphysiologicalfuellikeglucose(葡萄糖)intotheenergythatthecellsneedtofunction.Eventually,theyshutdown.
ButinapaperpublishedinthejournalCell,Sinclairandhiscolleaguesdescribeforthefirsttimeacompoundnaturallymadebyyoungcellsthatisabletomakeoldercellsenergeticandyouthfulagain.Inanexperimentinmice,theteamfoundthatgivingoldermiceachemicalcalledNADforjustoneweekmade2-year-old-micetissueresemblethatof6-month-oldmice(inhumanyears,thatwouldbelikea60-year-old'scellsbecomingmorelikethosebelongingtoa20-year-olD．.
Asanimalsage,saysSinclair,levelsofNADdropby50%;withlessofthecompound,thecommunicationbetweenthecellanditsmitochondrialenergysourcealsopausesoccasionally,andthecellbecomeseasilyhurtfromcommonagingattacks--inflammation(炎症),musclewasting(肌肉萎缩)andslowermetabolism(新陈代谢).Bytrickingthecellintothinkingit'syoungagain,withadequateamountsofNAD,agingcantheoreticallybereversed.
HisnextstepistoputNADinthedrinkingwaterofhismice,andseeiftheytakelongertodevelopthetypicalchronicdiseaseslinkedtoaging,suchasinflammation,musclewastingandcancer.Thepathwaymaybecomeanimportanttargetforcancerresearchersaswell,sincetumors(肿瘤)typicallygrowinlow-oxygenconditionsandaremorecommoninolderpatients.
BecauseNADisanaturallyoccurringcompoundthatsimplydeclineswithage,Sinclairisoptimisticthatboostingitslevelsinpeoplewon'thaveasmanysignificantnegativeeffectsasintroducinganentirelynewcompoundmight."Ifabodyisslowlyfallingapartandlosingtheabilitytoregulateitselfeffectively,wecangetitbackontracktowhatitwasinits20sand30s,"hesays.
Whatwillnotmakecellsage? A．Oxygen.
B．Wearandtear.
C．Lackofoxygen.
D．Lessphysiologicalfuel.
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