水体自净作用的概念是

A．水污染后可通过水体自身的物理、化学、生物学作用，将污染浓度逐渐降至污染前的状况

B．水污染后可通过污水处理厂进行净化，使之达到污染前状况

C．投入相应的净化剂，使污染物分解掉

D．投入大量需氧菌，使污染物分解掉

E．污水经综合利用后再排出

At a basic level,cloud computing is simply a means of delivering ITresources as ____(71)___.Almost all IT resources can be belivered as a cloud service:applications,compute power,storage capacity,networking,programming tools,even communication services and collaboration ____(72)____. Cloud computing began as large-scale Internet service providers such as Google,Amazon,and others built out their infrastructure.Anarchitecture emerged:massively scaled,____(73)____ distributed system resources,abstracted as virtual IT services and managed as continuously configured,pooled resources.In this architecture,the data is mostly resident on ___(74)___ "somewhere on the Internet" 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 ___(75)___ 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 of resources available - file storage,databases,and Web services - and extend the applicability to Web and enterprise applications.

（75）处填（）。

A．entire

B．individual

C．general

D．separate