prep-1-16, 答案有点怪

Most of the world's supply of uranium currently comes
from mines.  It is possible to extract
uranium from seawater, but the cost of doing so is greater than the price that
uranium fetches on the world market. 
Therefore, until the cost of extracting uranium from seawater can
somehow be reduced, this method of obtaining uranium is unlikely to be
commercially viable.

Which of the following would it be most useful to
determine in evaluating the argument?

(A) Whether the uranium in deposits on land is rapidly
being depleted

(B) Whether most uranium is used near where it is mined

(C) Whether there are any technological advances that
show promise of reducing the cost of extracting uranium from seawater

(D) Whether the total amount of uranium in seawater is
significantly greater than the total amount of uranium on land

(E) Whether uranium can be extracted from freshwater at a
cost similar to the cost of extracting it from seawater

答案是A

ding

show promise of reducing the cost is not real cost reduction.

I chose C too.

我也选错了，

我后来觉得题目最后结论说只有成本下降，海水提取新方法才会进行商业上的使用。如果针对结论作评价，对选项的回答可以进行加强和削弱的判断。

如果现在即使没有新科技，若陆地资源用尽，海水提取还是会被使用的，即使成本高。

但是C选项感觉只有加强。

Most of the world's supply of uranium currently comes
from mines.  It is possible to extract
uranium from seawater, but the cost of doing so is greater than the price that
uranium fetches on the world market. 
Therefore, until the cost of extracting uranium from seawater can
somehow be reduced, this method of obtaining uranium is unlikely to be
commercially viable.

Which of the following would it be most useful to
determine in evaluating the argument?

(A) Whether the uranium in deposits on land is rapidly
being depleted

(B) Whether most uranium is used near where it is mined

(C) Whether there are any technological advances that
show promise of reducing the cost of extracting uranium from seawater

(D) Whether the total amount of uranium in seawater is
significantly greater than the total amount of uranium on land

(E) Whether uranium can be extracted from freshwater at a
cost similar to the cost of extracting it from seawater

答案是A

i choose c too .why?

please give us more clear answer....3q

题目问的是evaluate,因此必定是对之前各部分条件的一个比较和深入阐述，而非重复题目中已经说明的信息。技术进步等等作为降低海洋提炼成本的因素已经在题目中考虑进去了。而原来不够严谨的是，假如陆地的铝还有很多，那么即使提炼成本稍微下降，还是要依赖于挖矿。

举个例子说明比较清楚，我们假定现在采矿成本为5每吨，而海水提炼为10每吨，如果选C,则有两种可能，一是海水提炼成本被削减到5或5以下，一是成本只能被削减到7－8左右，这两种可能行会对海水提炼的商业应用产生截然不同的可行性影响，因此我们还是没办法形成清晰的判断。

而出现选项A的情况，则说明必须从海水中提炼，哪怕成本再高，因为已经没有可替代的方法。在这种情形下，判断是非常清楚而且符合逻辑的。

C说的是现在有没有很有前途的能使得从海水提取铀的成本降低的科技

即使有，能否真的成为现实也是问题，现在没有就更没有讨论的意义，讨论一个现实意义不大的东西，对以evaluate“除非成本下降否则从海水提取铀没有商业价值”的结论没有价值。

注意题中的结论是：只有当从海水中提纯U的cost降下来，这种方法才有商业价值。

要否定的话应该是：不一定到这个时候，也有价值。

C只是说是否有这种技术能保证cost下降。相对于原结论是无关的。