Question

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.

Answer 1

- It just says "Leaching had been a method, but until 25 years ago," we didn't know a ... century, but until about 25 years ago, miners did not realize that bacteria take an .... "Leaching as a method of the extraction of minerals well established as early as ... Clause 2: it was well established as early as the eighteenth century,