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题库 / Prep2008E2-SC-196
Leaching, the recovery of copper from the drainage water of mines, as a method of the extraction of minerals, it was well established as early as the eighteenth century, but until about 25 years ago miners did not realize that bacteria take an active part in the process.
as a method of the extraction of minerals, it was well established
as a method of the extraction of minerals well established
was a well-established method of mineral extraction
was a well-established method of extracting mineral that was
had been a method of mineral extraction, well established
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又错选了e；
考察：
过去时态
误区：句中确实有完成时标志词until，但是这个是but后面的分句，与前面的句子直接加until还不一样；这里确实出现了过去时间点25年前，前面的句子也确实在25年前之前，但是这个没用过完，因为过去完成时表示leaching是过去的办法，现在不用了，但句中并没有表述这种意识；也就是but后面的句子无关leaching这个主语，基本是无关内容。如果but后面说这个方法leaching 25年前不用了，就不一样。
a well-established method > a method of mineral extraction, well established
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