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
(D) Importantly, in this sort of structure (NOUN1 + prep + NOUN2 + THAT..., or NOUN1 + (modifier ending with NOUN2) + THAT...), the "that"-modifier can actually modify either NOUN1 or NOUN2.
for instance:
the library has a new method of sorting cd's that makes certain genres of music easier to find. --> here, "that makes..." refers to the method of sorting.
the library has a new method of sorting cd's that don't fit into any of the traditional musical genres. --> here, "that don't fit..." refers to the cd's, not the method.
both sentences are correct.
however, in the example above, either of these two interpretations leads to an absurd sentence -- neither the method (= NOUN1) nor the mineral (= NOUN2) "was as early as the 18th century" -- so this option is flat-out incorrect.
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