Cloudcomputingisaphraseusedtodescribeavarietyofcomputingconceptsthatinvolvealargenumberofcomputers()throughareal-timecommunicationnetworksuchastheInternet．Inscience，cloudcomputingisa()fordistributedcomputingoveranetwork，andmeansthe(请作答此空)torunaprogramorapplicationonmanyconnectedcomputersatthesametime．Thearchitectureofacloudisdevelopedatthreelayers：infrastructure，platform，andapplication，Theinfrastructurelayerisbuiltwithvirtualizedcompute，storage，andnetworkresources．Theplatformlayerisforgeneral-purposeandrepeatedusageofthecollectionofsoftwareresources．TheapplicationlayerisformedwithacollectionofallneededsoftwaremodulesforSaaSapplications．Theinfrastructurelayerservesasthe()forbuildingtheplatformlayerofthecloud．Inturn，theplatformlayerisafoundationforimplementingthe()layerforSaaSapplications．
A.ability B.applroach C.function D.method正确答案A
