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16. the primary purpose of the passage is to (a) identify differing approaches (b) discount a novel trend (c) advocate traditional methods (d) correct misinterpretations (e) reconcile seeming inconsistencies since the early 1920s, most petroleum geologists have favored a biogenic theory for the formation of oil. according to this theory, organic matter became buried in sediments, and subsequent conditions of temperature (5) and pressure over time transformed it into oil. since 1979 an opposing abiogenic theory about the origin of oil has been promulgated. according to this theory, what is now oil began as hydrocarbon compounds within the earth s mantle (the region (10) between the core and the crust) during the formation of the earth. oil was created when gasses rich in methanc, the lightest of the hydrocarbons, rose from the mantle through fractures and fauhs in the crust, carrying a significant amount of heavier hydrocarbons with them. (15) as the gases encountered intermittent drops in pressure, the heavier hydrocarbons condensed, forming oil, and were deposited in reservoirs throughout the crust, rock regions deformed by motions of the crustal plates provided the conduits and fracures necessary for the (20) gases to rise through the crust. opponents of the abiogenic theory charge that hydrocarbons could not exist in the mantle, because high lemperatures would destroy or break them down. advocates of the theory, however, point out that other (25) types of carbon exist in the mantle: unoxidized carbon must exist there, because diamonds are formed within the mantle before being brought to the surface by eruptive processes. proponents of the abiogenic theory also point to recent experimental work that suggests (30) that the higher pressures within the mantle tend to offset the higher temperatures, allowing hydrocarbons, like unoxidized carbon, to continue to exist in the mantle. if the abiogenic theory is correct, vast undiscovered (35) reservoirs of oil and gas—undiscovened because the biogenic model precludes their existence—may in actuality exist. one company owned by the swedish government has found the abiogenic theory so persuasive that it has started exploratory drilling for gas (40) or oil in a granite formation called the siljan ring—not the best place to look for gas or oil if one belives they are derived from organic compounds, because granite forms from magma (molten rock) and contains no organic sediments. the ring was formed about 360 (45) million years ago when a large meteorite hit the 600-million-year-old granite that forms the base of the continental crust. the impact fractured the granite, and the swedes believe that if oil comes from the mantle, it could have risen with methane gas through this now (50) permeable rock. fueling their optimism further is the fact that prior to the start of drilling, methane gas had been detected rising through the granite. 17. which one of the following statements best expresses the main idea of the passage? (a) although the new abiogenic theory about the origin of oil is derived from the conventional biogenic theory, it suggests new types of locations for oil drilling. (b) the small number of drilling companies that have responded to the new abiogenic theory about the origin of oil reflects the minimal level of acceptance the theory has met with in the scientific community. (c) although the new abiogenic theory about the origin of oil fails to explain several enigmas about oil reservoirs, it is superior to the conventional biogenic theory. (d) although it has yet to receive either support or refutation by data gathered from a drilling project, the new abiogenic theory about the origin of oil offers a plausible alternative to the conventional biogenic theory. (e) having answered objections about higher pressures in the earth s core, proponents of the new abiogenic theory have gained broad acceptance for their theory in the scientific community. 18. which one of the following best describes the function of the third paragraph? (a) it presents a view opposed to a theory and points out an internal contradiction in that opposing view. (b) it describes a criticism of a theory and provides countervailing evidence to the criticism. (c) it identifies a conflict between two views of a theory and revises both views. (d) it explains an argument against a theory and shows it to be a valid criticism. (e) it points out the correspondence between an argument against one theory and arguments against similar theories.   |
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