At a basic level， cloud computing is simply a means of delivering IT resources as （）.Almost all IT resources can be delivered as a cloud service: applications， compute power， storage capacity， networking，programming tools，even communication services and collaboration （）。
Cloud computing began as large-scale Internet service providers such as Google，Amazon，and others built out their infrastructure. An architecture emerged: massively scaled， （） distributed system resources，abstracted as virtual IT services and managed as continuously configured， pooled resources. In this architecture， the data is mostly resident on （）“somewhere on the Intemet”and the application runs on both the "cloud servers”and the user’s browser.
Both clouds and grids are built to scale horizontally very efficiently. Both are built to withstand failures of （） elements or nodes. Both are charged on a per-use basis. But while grids typically process batch jobs， with a defined start and end point， cloud services can be continuous. What’s more， clouds expand the types ofresources available —— file storage， databases， and Web services —— and extend the applicability to Web and enterprise applications.
In this architecture， the data is mostly resident on （）“somewhere on the Intemet”and the application runs on both the "cloud servers”and the user’s browser.
A．clients
B．middleware
C．servers
D．hard disks