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.
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
SPOILER: Official Answer:C
why not E? because “Leaching” is happened earlier than the second part of the statement. Can anybody explain por favor, el señor?
eliminate a n b for using as-> as cant introduce phrase
well established shud come b4 method so tht reader can come to know that what exactly is well established..
d-> relative clause introduced by that is nt required...
e-> pst perfect nt required....