Cloudcomputingisaphraseusedtodescribeavarietyofcomputingconceptsthatinvolvealargenumberofcomputers_()__throughareal-timecommunicationnetworksuchastheInternet.Inscience,cloudcomputingisa_(请作答此空)__fordistributedcomputingoveranetwork,andmeansthe_()_torunaprogramorapplicationonmanyconnectedcomputersatthesametime.Thearchitectureofacloudisdevelopedatthreelayers:infrastructure,platform,andapplication.Theinfrastructurelayerisbuiltwithvirtualizedcompute，storage，andnetworkresources.Theplatformlayerisforgeneral-purposeandrepeatedusageofthecollectionofsoftwareresources.TheapplicationlayerisformedwithacollectionofallneededsoftwaremodulesforSaaSapplications.Theinfrastructurelayerservesasthe_()__forbuildingtheplatformlayerofthecloud.Inturn,theplatformlayerisfoundationforimplementingthe_()__layerforSaaSapplication.
A.replacement B.switch C.substitute D.synonym正确答案D
