单项选择题
A．was difficult to collect B．came in a variety of forms C．was difficult to get rid of D．tended to be absorbed by physical objects
A．method of concealing the source of heat B．special form of air conditioning C．supplementary hot water system D．way of reclaiming and re—using heat
A．The heat is supplied by human bodies only. B．The heat is supplied by both human bodies and other heatemitting objects. C．The heat is supplied by both human bodies and conventional fuel. D．The heat is supplied by human bodies, other heatemitting objects, and conventional fuel.