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

可否解释一下为何 C 对啊,句型实在是看不太出来,而A又是哪里出了问题呢

我觉得应该选E

划线部分后面紧跟着as early as the eighteenth century, 如果用C的话，非常的不顺畅，使得这部分显得nonsense.

而且，后面说到until about 25 years ago miners did not realize that bacteria take an active part in the process.就是说，Leaching法现在已经不用了，因此，E选项中用had been a method of mineral extraction非常贴切。

一家之言，欢迎批判.

Well, I think C is the best answer.

In E, "had been" indicate that leaching was method of mineral extraction until 25 years ago and stopped being so thereafter, while this is not the case. To express a general condition, the past tense "was" is preferred.

Just my understanding, open for criticism.