20. Which of the following techniques is used by the author in developing the argument in the passage?
Giving a clich a new meaning
Pointedly refusing to define key terms
Alternately presenting generalities and concrete details
Concluding the passage with a restatement of the first point made in the passage
Juxtaposing statements of what is not the case and statements of what is the case
The energy contained in rock within the earths 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 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 systemswhich involve drilling holes and connecting them to artificial reservoirs placed deep within the crustare 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.
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