(第2空)Cloud computing is a phrase used to describe a variety of computing concepts that involve a large number of computers ( ) through a real-time commutinication 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 infrastucture 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 infrastucture layer serves as the ( ) for building the platform layer of the cloud. In turn,the platform layer is foundation for implementing the ( ) layer for SaaS application.