Cloud computing is a phrase used to describe a variety of computing concepts that involve a large number of computers(  )through a real-time communication network such as the Internet. In science, 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 infrastructure layer is built with virtualized compute,storage,and network resources. The platform layer is for general-purpose and repeated usage of the collection of software resources.The application layer is formed with a collection of all needed software modules for SaaS applications. The infrastructure layer serves as the (  ) for building the platform layer of the cloud. In tum, the platform layer is a foundation for implementing the(  ) layer for SaaS applications.