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题库 / Prep2008E1-CR-82
Technological improvements and reduced equipment costs have made converting solar energy directly into electricity far more cost-efficient in the last decade. However, the threshold of economic viability for solar power (that is, the price per barrel to which oil would have to rise in order for new solar power plants to be more economical than new oil-fired power plants) is unchanged at thirty-five dollars.
Which of the following, if true, does most to help explain why the increased cost-efficiency of solar power has not decreased its threshold of economic viability?
The cost of oil has fallen dramatically.
The reduction in the cost of solar-power equipment has occurred despite increased raw material costs for that equipment.
Technological changes have increased the efficiency of oil-fired power plants.
Most electricity is generated by coal-fired or nuclear, rather than oil-fired, power plants.
When the price of oil increases, reserves of oil not previously worth exploiting become economically viable.
情景：技术进步和减少的设备成本让太阳能转换电能变得更有效率。但是，经济可行性阈值却完全没有变化。
推理：直接解释however身后的部分即可。
本题其实比较重要的问题是把什么叫经济可行性阈值看懂。阈值本身虽然是一种价格差，但是其本质是成本效益的比较。题目中所给出的经济可行性的阈值表示：“如果令某种方式在经济效益上超过火电，那么火电每桶油需要上升多少钱”。
选项分析：
A选项：油价下降了很多。在推理文段中，其实油价是一个量度的标准，真正比较的是经济效益。
B选项：尽管将太阳能转化为电能的器材原料价格变高，但是这种器材的成本还是下降了。本选项在解释前提中“减少的设备成本”。
C选项：Correct. 技术的革新增加了火电的经济效益。本选项直接谈及经济效益，可以有效的解释现象。
D选项：大部分的电力是来自于煤电和核电而不是火电的。本选项和待解释的现象无关。
E选项：当油价上涨的时候，有些以前不值得开发的储油地也开始变得值得开发了。本选项和待解释的现象无关。
economic viability经济可行性
只有C提到了efficiency of oil-fired，火电的经济效益
前提：太阳能的经济效率提高
结论：太阳能经济效率和火电一样（达到阈值）——题目是正常逻辑的反说，所以要解释这种反逻辑结论，就是削弱正逻辑
推理方式：因果
削弱：反驳正逻辑
题目的关键在“经济可行性阈值”这个概念，一个背景知识是：当前世界上大多数国家或者地区为了发展清洁能源，燃油发电的阈值需要提高，因为由于历史原因，目前燃油发电的成本依然是偏低的。
就这道题来说，我们假设发每单位的电，太阳能发电成本为100元，燃油发电实际成本需要65元，但国家为了发展太阳能，使它更具经济型，所以会使燃油发电成本提升超过太阳能发电，这里近似提升35元，那么这个提升的35元，就叫做“经济可行性阈值”。
如果太阳能发电成本降低，比如每单位电降低到90元，如果燃油发电成本不变，那么“经济可行性阈值”是要降低的（25元）
但是如果如C选项所说，燃油电厂的效率也提升了（成本降低），比如降低至每单位电55元，那么这时，经济可行性阈值是不变的，依然是35元，所以C选项正确。
Ron's explanation here: in this case, the problem is saying that solar power has gotten more efficient, but also that there has been no change in the target price of oil to achieve price equity with solar power. the only way this can be true is if something else related to oil-driven power, aside from the actual price of oil, has become cheaper in line with the improvements in solar power.
光电成本=火电成本+价格阀值（35）
由于前文提到科技进步使 光电成本 下降
那么在阀值一定的情况下，一定是 火力成本 相应下降了
但
燃料价格仅仅是火力成本的一个微小部分，其增减能体现但不能完全等同火电成本（包括除燃料价格之外的机电费用、管理费用等）的增减
且答案c引用了efficency与原文形成了良好的对应
因此c引用了原文的比较对象，使等式两边相对更加一致
在能选c的情况下，优选c
误选A the price per barrel to which oil would have to rise in order for new solar power plants to be more economical than new oil-fired power plants，题目说的是涨到多少到少
不是涨了多少多少！！！！
From Chasedream: A选项混淆了概念cost of oil和efficiency of oil-fired power。油价可以低不过不能保证efficiency高，因为可能管道维修贵了，石油工人工会工资问题......，只是举2个例子。意思说cost of oil和efficiency of oil-fired power的联系是主观的建立的，而不是题目已经给的。
光电成本=火电成本+价格阀值（35）
由于前文提到科技进步使 光电成本 下降
那么在阀值一定的情况下，一定是 火力成本 相应下降了
但
燃料价格仅仅是火力成本的一个微小部分，其增减能体现但不能完全等同火电成本（包括除燃料价格之外的机电费用、管理费用等）的增减
且答案c引用了efficency与原文形成了良好的对应
因此c引用了原文的比较对象，使等式两边相对更加一致
在能选c的情况下，优选c
questionable assumption 是可疑的假设的意思？？？？
A讲的仅仅是油的成本，与发电无关；C指的是油用于发电的效率提高，与题目更贴近。
可行性阈值是一种价格差 那么太阳能电的效率提高了 如果差值不变 火电的效率也必须提高
看懂这个阈值是关键。。但是真难看懂啊。。。
太阳能发电如果省钱了，也就是说发一度电的成本下降了，那么（原本调高了让两发电厂cost-efficiency一致的）油价应该下跌，以保证石油火力发电发一度电的成本与其保持一致；但问题是没有下跌。而这时，如果石油火力发电也变得更省钱了，也就是说发一度电不需要以前那么多油了，这时油价不变，同样也可以保证两者保持一致。
In a similar sense, the reason the threshold of viability has not changed in this problem has nothing to do with the price of oil, but instead has to do with how cost-effective solar power is relative to oil power. C does this by providing a scenario in which efficiency increases in solar power are balanced by efficiency increases in oil power.
A中成本下降，有可能利润上升，所以应该导致阈值变大。（成本与利润之间可能有关，但成本与收入之间并无绝对关系）
如果A选项改为成本上升，则利润下降，阈值就减小，此时有可能正确。
老师，我这样的理解对吗
第一遍没读懂题
错选了A，理解了题不够仔细，A是说油价， 但是题目说的是火电和太阳能发电的经济效益差别，所以应该抓住的是这两个发电的成本差异
In a similar sense, the reason the threshold of viability has not changed in this problem has nothing to do with the price of oil, but instead has to do with how cost-effective solar power is relative to oil power. C does this by providing a scenario in which efficiency increases in solar power are balanced by efficiency increases in oil power.
错选了E。错因：1.文中解释概念的那句话没看懂 2.排除法用的不好，抓住因与果涉及的对象就可以排除一部分选项
The question is essentially asking why the threshold of economic viability has remained unchanged. In other words, why would the price of oil still have to hit $35 in order for solar powered plants to be a good idea? The best answer I can give is that the price of oil has nothing to do with how much the price *would* have to be in order for solar power to be viable. What would change my threshold of viability? NOTHING about the price of the oil can change the threshold.
FYI https://www.manhattanprep.com/gmat/forums/cr-technological-improvements-and-reduced-equipment-costs-t7885.html