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 infrastucture layer is built with virtualized computestorageand network resources. The platform lay-er 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 infrastucture layer serves as the（请作答此空）for building the platform layer of the cloud. In turn, the platform layer is founda-tion for implementing the（ ）layer for SaaS application.
A.network
B.foundation
C.software
D.hardware
- A.见图A
- B.见图B
- C.见图C
- D.见图D
