Cloudcomputingisaphraseusedtodescribeavarietyofcomputingconceptsthatinvolvealargenumberofcomputers___71___throughareal-timecommunicationnetworksuchastheInternet.Inscience,cloudcomputingisa___请作答此处___fordistributedcomputingoveranetwork,andmeansthe___73___torunaprogramorapplicationonmanyconnectedcomputersatthesametime. Thearchitectureofacloudisdevelopedatthreelayers:infrastructure,platform,andapplication.Theinfrastucturelayerisbuiltwithvirtualizedcomputestorageandnetworkresources.Theplatformlay-erisforgeneral-purposeandrepeatedusageofthecollectionofsoftwareresources.TheapplicationlayerisformedwithacollectionofallneededsoftwaremodulesforSaaSapplications.Theinfrastucturelayerservesasthe___74___forbuildingtheplatformlayerofthecloud.Inturn,theplatformlayerisfounda-tionforimplementingthe___75___layerforSaaSapplication.
A.replacement B.switch C.substitute D.synonym（同义词）正确答案D
