单选题 The energy contained in rock within the earth"s crust represents a nearly unlimited energy source, but until recently commercial retrieval has been limited to underground hot water and/or steam recovery systems. These systems have been developed in areas of recent volcanic activity, where high rates of heat flow cause visible eruption of water in the form of geysers and hot springs. In other areas, however, hot rock also exists near the surface but there is insufficient water present to produce eruptive phenomena. Thus a potential hot dry rock (HDR) reservoir exists whenever the amount of spontaneously produced geothermal fluid has been judged inadequate for existing commercial systems.
As a result of recent energy crisis, new concepts for creating HDR recovery systems—which involve drilling holes and connecting them to artificial reservoirs placed deep within the crest—are being developed. In all attempts to retrieve energy from HDRs, artificial stimulation will be required to create either sufficient permeability or bounded flow paths to facilitate the removal of heat by circulation of a fluid over the surface of the rock.
The HDR resource base is generally defined to include crustal rock that is hotter than 150℃, is at depths less than ten kilometers, and can be drilled with presently available equipment. Although wells deeper than ten kilometers are technically feasible, prevailing economic factors will obviously determine the commercial feasibility of wells at such depths. Rock temperatures as low as 100℃ may be useful for space heating; however, for producing electricity, temperatures greater than 200℃ are desirable. The geothermal gradient, which specifically determines the depth of drilling required to reach a desired temperature, is a major factor in the recoverability of geothermal resources. Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tappable high-temperature gradients are distributed all across the United States. (There are many areas, however, for which no temperature gradient records exist.)
Indications are that the HDR resource base is very large. If an average geothermal temperature gradient of 22℃ per kilometer of depth is used, a staggering 13,000,000 quadrillion B.T.U. "s of total energy are calculated to be contained in crustal rock to a ten-kilometer depth in the United States. If we conservatively estimate that only about 0.2 percent is recoverable, we find a total of all the coal remaining in the United States. The remaining problem is to balance the economics of deeper, hotter, more costly wells and shallower, cooler, less expensive wells against the value of the final product, electricity and/or heat.

单选题 The primary purpose of the passage is to ______.

单选题 The passage would be most likely to appear in a ______.

单选题 According the passage, an average geothermal gradient of 22℃ per kilometer of depth can be used to ______.

【正确答案】 D

【答案解析】[解析] 本题为细节题。根据文中最后一段第一和第二句话“Indications are that the HDR resource base is very large. If an average geothermal temperature gradient of 22℃ per kilometer of depth is used, a staggering 13,000,000 quadrillion B.T.U. "s of total energy are calculated to be contained in crustal rock to a ten-kilometer depth in the United States.”可知，HDR的能源库非常之大。如果利用每深度公里的地热梯度平均为22℃，可以计算美国1万公里深度的地壳岩石中所包含的总能源。D选项“估算美国HDR能源库”符合原文意思，因此D项正确。

单选题 It can be inferred from the passage that the availability of temperature-depth records for any specific area in the United States depends primarily on the ______.

【正确答案】 B

【答案解析】[解析] 本题为分析题。根据文中第四段第二句话“Temperature gradient maps generated from oil and gas well temperature-depth records kept by the American Association of Petroleum Geologists suggest that tappable high-temperature gradients are distributed all across the United States.”可知，美国石油地质学家协会制定的石油和天然气的温深记录形成了温度梯度图。B选项“该地区之前钻取石油或天然气的尝试”符合原文意思，因此B项正确。