OG12 - 114求助

alexibm

今天读到这篇，也在这里栽了跟头。搜了解释发现还是不明白。看了dayuyunhai兄的提醒“吃透一篇”，又重新读了一遍，忽然觉得豁然开朗：

题目问的是methods of exploring for gold that are widely used “today ”，回到原文。看我下面截取的文章片段，然后看我用引号引起来的3个点，就会发现答案应该是C。

Most of the gold deposits discovered during the “original ”gold rushes were
exposed at the Earth's surface and were found
because they had shed trails of alluvial gold
(15) that were easily traced by simple prospecting
methods. “Although these same methods still lead
to an occasional discovery, most deposits not yet
discovered have gone undetected because they are
buried and have no surface expression”.
(20)    The challenge in exploration is therefore to
unravel the subsurface geology of an area and
pinpoint the position of buried minerals. Methods
widely used ”today “include