According the passage, "an average geothermal gradient of 22℃ per kilometer of depth" can be used to A. balance the economics of HDR energy retrieval against that of underground hot water or steam recovery systems B. determine the amount of energy that will used for space heating in the United States C. provide comparisons between hot water and HDR energy sources in United States D. revise the estimates on the extent of remaining coal resources in the United States E. estimate the total HDR resource base in the United States