求助一道阅读题（新人发帖，请包涵呀～）

melanietingtingUID：3565526

　　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 Line 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.

7. 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

答案是D，但是完全没头绪不知道为什么
请高手帮忙解答一下吧！

iwishicanUID：3155544

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. 这个D应该是从这里找原因吧。

iwishicanUID：3155544

我不是高手，共同学习一下哦。上面的大概意思就是不同矿物来源LEAD的isotopic composition 也不同，所以D说一样估计就是选项了。

melanietingtingUID：3565526

iwishican 发表于 2015-4-24 16:14
我不是高手，共同学习一下哦。上面的大概意思就是不同矿物来源LEAD的isotopic composition 也不同，所以D说 ...

哇，不好意思才发现回复。以为这个帖子就要沉了呢！！

万分感谢，我先回一个感谢，然后再细细看一下！

yuhaichao10UID：3574819

这题问的是哪个不会改变 resulting metal 中的锌的同位素的比重（lead isotope composition），显然只有D是对的，只有当加入的flux中的同位素比重和被加入的矿石中同位素比重相同时，才不会改变。

可以类比两个溶液，只有当A溶液的浓度和B溶液的浓度一样时，A加入B才不会改变B的浓度，这样说不知道你能不能理解？