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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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这边cd的区别是名词和ing的区别吗，是不是因为被建立了说明他是一个有时间截点的动作，所以选名词形式？？？
对，做某种事情的方法，一定隐含着这件事至少已经被完成一次，所以用名词形式
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