请教XDJM 2000.8月听力题第36题

part B一个关于地质的长对话，大家帮忙看看有道题不解：36题  what is the discussion mainly about?
A.How rocks are eroded
B. How sediment hardens into rock
C. How flood plains develop
D. How minerals are extracted from rock
答案选A，而觉得应该选B

对话如下：
Man: let's say you are geologist, and u want to investigate the geological history of a place. That is, how did geologists determine things like. . . Say. . How were the rocks formed? Or was an area once under water? If so, when? How should u about it?  
Woman: I’d start with stratigraphy.  
Man: could u explain what this is to the class?  
Woman: well, stratigraphy is the description of strata in sedimentary rock. I guess that's not so clear, huh? Ok, let's say one of the investigators thought near a river, for example. Well, over the history of the area, every time the river flooded, it would deposit a layer of sediment all through with flooded, it would deposit a layer of sediment all through with floodplain. Sometimes a bigger layer, sometimes a smaller, depending on the size of the flood. Well, one layer or stratum gets deposited over another. Obviously these strata built up over millions of years. Stratigraphy is the study of these layers of deposited settlement.  
Man: so does that mean if I examine each of these strata, I can tell how long ago each one is deposited?  
Woman: not necessarily. U see, there might've been some years when the river didn't flood and no settlement was deposited. You need other kinds of evidence to tell how much time might've gone by between when one layer got deposited and one on top of it got deposited.  
Man: and what are those other kinds of evidence u are talking about?  
Woman: well, fossils for one. U can determine exactly how old a fossil is and that's how u can tell how the rock surrounding it is.  
Man: very good. The discovery of that particular technique is an interesting story. It was a man named William smith who first used fossils for the purpose of dating strata back in the 1800s. Let's take a look at how he went about making this geological breakthrough.

主要是erode这个词用在这里有点迷惑，一般感觉是侵蚀，不过这里却是描述沉积。但文章的主要意思是讨论stratum怎么形成和怎么知道年代长久。还有就是从第一句第二行可以看出来，How were the rocks formed?

第二个答案中的harden是不对的，文章中没有说到相关的内容。A稍微好一些吧。。

不过最好有地质方面专长的朋友给我介绍一下这些专用词的表述。。呵呵