Cloud computing is a phrase used to describe a variety of computing concepts that involve a large number of computers (1) through a real-time communication network such as the Internet. In science, cloud computing is a (2)for distributed computing over a network, and means the (3) 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 (4)for building the platform layer of the cloud. In turn, the platform layer is a foundation for implementing the (5) layer for SaaS applications.
空白（2）处应选择（）
A.replacement
B.switch
C.substitute
D.synonym（同义词）