单选题 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 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.