(第1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 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 turn，the platform layer is a foundation for implementing the( )layer for SaaS applications．