信息系统开发中，由程序员完成程序的编写与调试、由系统分析员培训业务人员和信息系统操作人员、由业务人员安装与调试计算机等设备、为新信息系统的运行完成数据文件的转换或生成等任务属于______。

A．系统分析阶段

B．系统设计阶段

C．系统实施阶段

D．系统运行和维护阶段

Air pollution is caused by many things. Great clouds of smoke come from factories. Cars send smoke and fumes into the air. Burning waste fills the air with bits of dirt. Even burning leaves add smoke to the air. Sometimes, planes cannot find their landing spots through smoke. Smoke of all kinds is bad to breathe and can do great harm to the body.
Still, we cannot live without air. We must find a way to clean the air we have.
In time, we may have factories that are run by atomic energy. Our cars may run on smoke-free electric power. Scientists are also working on new ways to help the oil burned by cars from making fumes. Waste may be burned in the sand on ocean floors.
These changes might keep our air clean in the years to come. But until then, many scientists are looking for ways to make air cleaner now.

Scientists are now working on ways to ______.

A．keep planes from :flying

B．bury sand on ocean floors

C．stop air pollution

D．burn leaves without smoke