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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
请看讲解视频
C和D之间如何选？如果分不清extraction和extracting的话，可以这么看，D中的that was无论是指代method还是mineral都使得后面的as early as变成从句的成分，那就变成了method（或者mineral）was as early as the eighteenth century，显然说不通。再看C中as early as the eighteenth century是一个时间状语，很完美！
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