Cloud computing is a phrase used to describe a variety of computing concepts that involve a large number of computers_（请作答此空）__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 foundation for implementing the_（ ）__layer for SaaS application.
A.connected
B.implemented
C.optimized
D.virtualized