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 towithstand 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 servics can be continuous. What's more, clouds expand the types of resources 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

写出下列反应的符号表达式，并回答有关问题．

示例：硫在氧气中燃烧   S+O₂

（1）镁条在空气中燃烧：______；在此实验中夹持镁条的仪器是______．

（2）用双氧水和二氧化锰制取氧气：______；其中二氧化锰起______作用．

（3）纯净的氢气在空气中安静燃烧：______；该反应的基本类型是______反应．

（4）碳酸氢铵受热分解的反应：______；如何保存碳酸氢铵？______．