[求助]GWD-1-31

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.

1. as a method of the extraction of minerals, it was well established
2. as a method of the extraction of minerals well established
3. was a well-established method of mineral extraction
4. was a well-established method of extracting mineral that was
5. had been a method of mineral extraction, well established

answer is C.

我怎么看不懂啊？我把句子简化一下：

Leaching was a method as early as the eighteenth century.....

这里把 method 和 eighteenth century 在做比较吗？

哪位NN帮个忙吧。

自己答一下自己的问题。

结构︰as many／much as＋数字＋复数名词／不可数名词
说明︰此句型意为“多达…”。不要与“as many／much＋<名词>”（同数量的…）混淆。同理，此类<词组>还有：as long as＋数字＋<名词>（长达…），as wide as＋数字＋<名词>（宽达…）， as high as＋数字＋<名词>（高达…），as early as＋数字＋<名词>（早在…），as late as＋数字＋<名词>（迟在…）等。
    She has as many as seven sisters.  她的姊妹有七人之多。
    As many as 100 people were killed in the air crash. 这次空难中多达一百人丧生。
    I gave the beggar as much as 1,000 yen.   我给了这乞丐整整一千元。
    Building costs ran up to as much as NT$ 20 million.  建筑费用涨到高达新台币两千万。
    I have been learning English (for) as long as 15 years. 我学英文已有十五年之久。
    He came home as late as two in the morning.  他迟至清晨两点才回家。
    I saw your brother as late as last week. 上个礼拜我还看到你哥哥。
    The river is as wide as 100 meters.  这条河宽达一百公尺。
    I mailed the letter as early as Friday.  我早在星期五就把信寄了。