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

A. 见图A

B. 见图B

C. 见图C

D. 见图D