Cloud computing is a phrase used to describe avariety of computing concepts that involve a large number of computers （ ） through a real-time communication network such as the Internet. Inscience,cloud computing is a （ ） for distributed computing over a network, and means the （ ） to run a program or application on many connected computers at the same time.
The architecture of a cloud is developed at three layers: infrastructure, platform, and application. The infrastucture layer is built with virtualized compute, storage, andnetworkresources.Theplatformlayerisforgeneral-purposeandrepeatedusageofthecollectionofsoftwareresources.TheapplicationlayerisformedwithacollectionofallneededsoftwaremodulesforSaaSapplications.Theinfrastucturelayerservesasthe（ ）forbuildingtheplatformlayerofthecloud.Inturn,theplatformlayerisfoundationforimplementingthe（ ）layerforSaaS applications.（ ）