GRE阅读练习题及答案（31）

　　The determination of the sources of copper ore used in the manufacture of copper and bronze artifacts of Bronze Age civilizations would add greatly to our knowledge of cultural contacts and trade in that era. Researchers have analyzed artifacts and ores for their concentrations of elements, but for a variety of reasons, these studies have generally failed to provide evidence of the sources of the copper used in the objects. Elemental composition can vary within the same copper-ore lode, usually because of varying admixtures of other elements, especially iron, lead, zinc, and arsenic. And high concentrations of cobalt or zinc, noticed in some artifacts, appear in a variety of copper-ore sources. Moreover, the processing of ores introduced poorly controlled changes in the concentrations of minor and trace elements in the resulting metal. Some elements evaporate during smelting and roasting; different temperatures and processes produce different degrees of loss. Finally, flux, which is sometimes added during smelting to remove waste material from the ore, could add quantities of elements to the final product.
　　An elemental property that is unchanged through these chemical processes is the isotopic composition of each metallic element in the ore. Isotopic composition, the percentages of the different isotopes of an element in a given sample of the element, is therefore particularly suitable as an indicator of the sources of the ore. Of course, for this purpose it is necessary to find an element whose isotopic composition is more or less constant throughout a given ore body, but varies from one copper ore body to another or, at least, from one geographic region to another.
　　The ideal choice, when isotopic composition is used to investigate the source of copper ore, would seem to be copper itself. It has been shown that small but measurable variations occur naturally in the isotopic composition of copper. However, the variations are large enough only in rare ores; between samples of the common ore minerals of copper, isotopic variations greater than the measurement error have not been found. An alternative choice is lead, which occurs in most copper and bronze artifacts of the Bronze Age in amounts consistent with the lead being derived from the copper ores and possibly from the fluxes. The isotopic composition of lead often varies from one source of common copper ore to another, with variations exceeding the measurement error; and preliminary studies indicate virtually uniform isotopic composition of the lead from a single copper-ore source. While some of the lead found in an artifact may have been introduced from flux or when other metals were added to the copper ore, lead so added in Bronze Age processing would usually have the same isotopic composition as the lead in the copper ore. Lead isotope studies may thus prove useful for interpreting the archaeological record of the Bronze Age.
　　21. The primary purpose of the passage is to
　　(A) discuss the techniques of analyzing lead isotope composition
　　(B) propose a way to determine the origin of the copper in certain artifacts
　　(C) resolve a dispute concerning the analysis of copper ore
　　(D) describe the deficiencies of a currently used method of chemical analysis of certain metals
　　(E) offer an interpretation of the archaeological record of the Bronze Age
　　22. The author first mentions the addition of flux during smelting (lines 18-21) in order to
　　(A) give a reason for the failure of elemental composition studies to determine ore sources
　　(B) illustrate differences between various Bronze Age civilizations
　　(C) show the need for using high smelting temperatures
　　(D) illustrate the uniformity of lead isotope composition
　　(E) explain the success of copper isotope composition analysis
　　23. The author suggests which of the following about a Bronze Age artifact containing high concentrations of cobalt or zinc?
　　(A) It could not be reliably tested for its elemental composition.
　　(B) It could not be reliably tested for its copper isotope composition.
　　(C) It could not be reliably tested for its lead isotope composition.
　　(D) It could have been manufactured from ore from any one of a variety of sources.
　　(E) It could have been produced by the addition of other metals during the processing of the copper ore.
　　24. According to the passage, possible sources of the lead found in a copper or bronze artifact include which of the following?
　　I. The copper ore used to manufacture the artifact
　　II. Flux added during processing of the copper ore
　　III. Other metal added during processing of the copper ore
　　(A) I only
　　(B) II only
　　(C) III only
　　(D) II and III only
　　(E) I, II, and III
　　25. The author rejects copper as the “ideal choice” mentioned in line 33 because
　　(A) the concentration of copper in Bronze Age artifacts varies
　　(B) elements other than copper may be introduced during smelting
　　(C) the isotopic composition of copper changes during smelting
　　(D) among common copper ores, differences in copper isotope composition are too small
　　(E) within a single source of copper ore, copper isotope composition can vary substantially
　　26. The author makes which of the following statements about lead isotope composition?
　　(A) It often varies from one copper-ore source to another.
　　(B) It sometimes varies over short distances in a single copper-ore source.
　　(C) It can vary during the testing of artifacts, producing a measurement error.
　　(D) It frequently changes during smelting and roasting.
　　(E) It may change when artifacts are buried for thousands of years.
　　27. It can be inferred from the passage that the use of flux in processing copper ore can alter the lead isotope composition of the resulting metal EXCEPT when
　　(A) there is a smaller concentration of lead in the flux than in the copper ore
　　(B) the concentration of lead in the flux is equivalent to that of the lead in the ore
　　(C) some of the lead in the flux evaporates during processing
　　(D) any lead in the flux has the same isotopic composition as the lead in the ore
　　(E) other metals are added during processing