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  3. Potash (the old name for potassium carbonate) is one of the two alkalis (the other being soda, sodium carbonate) that were used

Potash (the old name for potassium carbonate) is one of the two alkalis (the other being soda, sodium carbonate) that were used

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问题       Potash (the old name for potassium carbonate) is one of the two alkalis (the other being soda, sodium carbonate) that were used from remote antiquity in the making of glass, and from the early Middle Ages in the making of soap: the former being the product of heating a mixture of alkali and sand, the latter a product of alkali and vegetable oil. Their importance in the communities of colonial North America need hardly be stressed. Potash and soda are not interchangeable for all purposes, but for glass or soap making either would do. Soda was obtained largely from the ashes of certain Mediterranean sea plants, potash from those of inland vegetation. Hence potash was more familiar to the early European settlers of the Noah American continent.
      The settlement at Jamestown in Virginia was in many ways a microcosm of the economy of colonial North America, and potash was one of its first concerns. It was required for the glassworks, the first factory in the British colonies, and was produced in sufficient quantity to permit the inclusion of potash in the first cargo shipped out of Jamestown. The second ship to arrive in the settlement from England included among its passengers exports in potash making.
      The method of making potash was simple enough. Logs was piled up and burned in the open, and the ashes collected. The ashes were placed in a barrel with holes in the bottom, and water was poured over them. The solution draining from the barrel was boiled down in iron kettles. The resulting mass was further heated to fuse the mass into what was called potash. In North America, potash making quickly became an adjunct to the clearing of land for agriculture, for it was estimated that as much as half the cost of clearing land could be recovered by the sale of potash. Some potash was ex- ported from Maine and New Hampshire in the seventeenth century, but the market turned out to be mainly domestic, consisting mostly of shipments from the northern to the southern colonies. For de- spite the beginning of the trade at Jamestown and such encouragements as a series of acts "to encourage the making of potash," beginning in 1707 in South Carolina, the softwoods in the South proved to be poor sources of the substance.  
What aspect of potash does the passage mainly talks about?
选项 A、How it was made.
B、Its value as a product for export.
C、How it differs from other alkalis.
D、Its importance in colonial North America.
答案D
解析 本题是一个主题题，考查考生对文章主题大意的理解。考生需要结合文章的体裁来回答这类问题。
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