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，theplatformlayerisafoundationforimplementingthe()layerforSaaSapplications．
A.replacement
B.switch
C.substitute
D.synonym(同义词)
正确答案是D