Every human being, no matter what he is doing, gives off body heat. The usual problem is how to dispose of it. But the designers of the Johnstown campus of the University of Pittsburgh set themselves the opposite problem－how to collect body heat. They have designed a collection system which utilizes not only body heat, but the heat given off by such objects as light bulbs and refrigerators as well. The system works so well that no conventional fuel is needed to make the campus's six buildings comfortable.

Some parts of most modern buildings－theaters and offices as well as classrooms are more than amply heated by people and lights and sometimes must be air-conditioned even in winter. The technique of saving heat and redistributing it is called “heat recovery.” A few modern buildings recover heat, but the University's system is the first to recover heat from some buildings and reuse it in others.

Along the way, pit has learned a great deal about some of its heat producers. The harder a student studies, the more heat his body gives off. Male students emit more heat than female students, and the larger a student, the more heat the produces. It is tempting to conclude that the hottest prospect for the Johnstown campus would be a hardworking, overweight male genius.

Until recently, body heat has caused problems because it____.（　　）

• 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