Leaching

Once reading through the stimulus, it is obvious that Leaching is the subject of the sentence. The way this particular stimulus is written leaching is followed by two modifiers, both meaning to modify leaching, but when you use two consecutive modifiers to modify the same thing the result is (as it is in the question stem) often ambiguous.

Leaching in this sentence is immediately followed by the modifier, " the recovery of copper from the drainage of water pipes" and is followed by another modifier, "as a method of extraction of minerals." You have to ask your self, what exactly is the second modifier modifying; leaching or the draining of water mines? Thus, the answer will clear up the leaching modifiers correctly.

More importantly, every sentence has a subject and an action. The way the sentence is written you can conclude that leaching is the subject, but the sentence has no action. Leaching has to be doing something. This is why you can say that "as a method of extraction" is not correct. This knocks out A and B.

Now is it "was" or "had been", (C,D) or (E)?

When you use the phrase "had been" it implies that at some other point you will discuss how what had been no longer is. But the sentence does not go on to talk about how leaching is no longer a method of extracting materials. It talks about how bacteria was added to this method of extracting. So we can conclude that "was" should be used. This verb should be in the simple past tense. E is out.

Now, is it C or D; "mineral extraction" or "extracting mineral that was".

Lets consider the latter,

"was a well-established method of extracting mineral that was as early as eighteenth century" Take out the modifiers it would read;

"was a method that was as early as eigtheenth century" How can a method be as early as anything. This sentence is nonsensical.

You can knock out D. C is the correct answer.

There is nothing wrong with was a well-established method of mineral extraction being behind as early as is perfectly fine grammatically.