Cloudcomputingisaphraseusedtodescribeavarietyofcomputingconceptsthatinvolvealargenumberofcomputers()throughareal-timecommunicationnetworksuchastheInternet.Inscience,cloudcomputingisa(请作答此空)fordistributedcomputingoveranetwork,andmeansthe()torunaprogramorapplicationonmanyconnectedcomputersatthesametime.Thearchitectureofacloudisdevelopedatthreelayers:infrastructure,platform,andapplication.Theinfrastucturelayerisbuiltwithvirtualizedcomputestorageandnetworkresources.Theplatformlay-erisforgeneral-purposeandrepeatedusageofthecollectionofsoftwareresources.TheapplicationlayerisformedwithacollectionofallneededsoftwaremodulesforSaaSapplications.Theinfrastucturelayerservesasthe()forbuildingtheplatformlayerofthecloud.Inturn,theplatformlayerisfounda-tionforimplementingthe()layerforSaaSapplication.
A.replacement B.switch C.substitute D.synonym（同义词）正确答案D
