As there is less and less coal and oil, scientists are exploring new ways of making use  of ______________   energy, such as sunlight, wind and water for power and fuel.

A．abrupt

B．alternative

C．authentic

D．abstract

供热管网可根据不同原理进行分类。其中根据（）划分，热网可分为区域式和统一式两类。

A.热源与管网之间的关系

B.输送介质的不同

C.平面布置类型

D.用户对介质的使用情况