题目内容：
Questions are based on the following passage．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 also 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 modem 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 modem buildings recover heat, but the University's system is the first to recover heat from some buildings and re-use it in others．
Along the way, Pitt has learned a great deal about some of its heat producers．The harder a student studies, the more heat his body gives off．(80) Male students emit more heat than female students do, and the larger a student is, the more heat he 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
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