(A) as a method of the extraction of minerals, it was well established
(B) as a method of the extraction of minerals well established
(C) was a well-established method of mineral extraction
(D) was a well-established method of extracting mineral that was
(E) had been a method of mineral extraction, well established
Solution:
Subject verb agreement + Modifier
A and B: a verb is missing in the first half of the sentence to make it a clause and hence, is incorrect. Leaching, , as a , it was..
D: that refers should refer to method but here it seems to be referring to mineral which is incorrect. "that was as early as" - wrongly suggests that the mineral was as early as 18 century.
E: misplaced modifier - is it the "mineral extraction" or the "method" which is "well-established"? Further had been wrongly suggests that leaching is no longer a method of mineral extraction
C: Best choice.
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