请教蓝皮书中的一题，NN进来看一下

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

i choose E,but the answer is C

what's wrong with E?

..., but until about 25 years ago...
        

可能until对你判断产生影响，但逗号+but已经引出一个完整的独立句了，且用了did not realize... 所以要说对称，要看这里啊！

另外选E不对的地方还有: well established 就近修饰mineral extraction，不符原意。

看着思故mm 在一天天进步呢。。。呵呵。。