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 computer, 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 turn, the platform layer is a foundation for implementing the（ ）layer for SaaS applications.