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 （请作答此空） 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.replacement
B.switch
C.substitute
D.synonym

A. 见图A

B. 见图B

C. 见图C

D. 见图D