As mines age, ore grades decline, since the best stuff is usually dug up first.

随着矿山年龄的增长，铁矿石的等级下滑，因为最好的往往被最先发掘。

youdao

As mines age, ore grades decline, since the best stuff is usually dug up first.

随着矿山年龄的增长，铁矿石的等级下滑，因为最好的往往被最先发掘。

youdao