解释题：没整明白，求解。。。

dq81

还有一题，答案是C。我没整明白，有nn愿解释下么？谢谢！
82.(33511-!-item-!-188;#058&007598)



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?

A. The cost of oil has fallen dramatically.

B. The reduction in the cost of solar-power equipment has occurred despite increased raw material costs for that equipment.

C. Technological changes have increased the efficiency of oil-fired power plants.

D. Most electricity is generated by coal-fired or nuclear, rather than oil-fired, power plants.

E. When the price of oil increases, reserves of oil not previously worth exploiting become economically viable.

dq81

么人理我，自己顶一下

andreasyang

hand。。。今天我也做到这题了，连题目都没搞懂。括号里的那段在讲什么啊。。。

toutakuu

原文不是说了the threshold of economic viability for solar power 是相对oil来讲的，那么技术的发展使the efficiency of oil-fired power plants增长，自然就解释了为什么threshold of economic viability没有减少

dq81

谢谢楼上，这道题我从网上找了比较详细的解释，大家可以参考一下：
I originally put A as well, but I now realize that the answer is C. I hope the following explanation will help.

The question here asks us to explain why the threshold of economic viability has not decreased from $35, despite technological improvements in solar power equipment. The key to this question is to realize what the definition of economic viability is, as defined by the GMAT writers of this question. This specific definition can be found in the parenthesis in the question, the part where it says "(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)" This is where the "threshold of economic viability" is defined. Reading this carefully, you will realize that the "price of oil" is a threshold that will not change unless the ratio of efficiencies of oil power equipment vs. solar power equipment increases or decreases. This is the ONLY thing that will change the threshold.

The question states that the efficiency of solar power equipment has increased. If the efficiency of oil power equipment has stayed the same, then the ratio of efficiencies of solar vs. oil power equipment should change. However, the anomaly the question is asking about is why the ratio has not changed. The ONLY way the ratio won't change despite the increased efficiency of solar power equipment is if the efficiency of oil power equipment increased by the same amount as the efficiency of solar power equipment. For those who think more in terms of math: If the solar power equipment efficiency was originally "100" and the oil power equipment efficiency was originally "200", then the ratio of "old solar : old oil" is 100:200, or 1/2. Now, the solar power equipment efficiency increased, so let's say its efficiency is now "200". If the oil power equipment efficiency remained the same as before, the ratio of "new solar : old oil" would now be 200:200, or 1, which is a change from the original old ratio of 1/2. The only way the ratio of new solar : old oil would remain at 1/2 is if oil power equipment's efficiency increased to "400" from its original 200, making the ratio of new solar : new oil equal to 200:400, or 1/2, which is the same as the old ratio. This clearly demonstrates that the answer is C. That's the only answer that discusses technological improvements in oil power equipments that make it more efficient .

The reason why Choice A is wrong is because the cost of oil has no effect on this $35 threshold. To understand this, I thought of the following two extreme scenarios:
1) Oil all of a sudden costs $1 million / barrel (or some other ridiculous amount of money), does this change the threshold? No. The threshold is still at $35. The fact that oil now costs $1 million / barrel means that the threshold of $35 is exceeded, which means it is now more efficient to use solar power equipment than oil power equipment. However, it doesn't mean the threshold of $35 has changed.
2) Oil all of a sudden becomes free. It costs $0. Does this effect the $35 threshold? No. The threshold is independent of the cost of oil. Even if oil is free, the threshold for solar power equipment to become more efficient than oil power equipment is still $35, because the efficiencies of the equipment do not depend on the price oil, they only depend on the technical abilities of the equipments. Now, you might say, doesn't that explain why the threshold of efficiency is unchanged? It does not. The reason why I showed two extreme scenarios is to show that the price of oil is IRRELEVANT to the threshold. If it CAN explain why the threshold of efficiency is unchanged, then one of the two extreme scenarios should change the threshold of $35. However, I have shown both extreme scenarios and neither of them effect the threshold of $35. Therefore, Choice (A) is irrelevant and is the wrong answer.

wyw1018

研究了此题，也参看了其它论坛一些解释，觉得都没有解释到位或者基本都错误，虽然答案正确，当然很多都是参考了正确答案然后再去反推的。
并不是threshold和oil price没有关系，相反是肯定有关系的。只是答案A只给出了适用油价下降这一单一原因，而答案C可以适用解释油价上涨，下降和不变的所有情形都可以使thrshold economic viability维持不变。
所以答案C比A好，而不是因为threshold和oil price没有关系。

shirryzhan

研究了此题，也参看了其它论坛一些解释，觉得都没有解释到位或者基本都错误，虽然答案正确，当然很多都是参考了正确答案然后再去反推的。
并不是threshold和oil price没有关系，相反是肯定有关系的。只是答案A只给出了适用油价下降这一单一原因，而答案C可以适用解释油价上涨，下降和不变的所有情形都可以使thrshold economic viability维持不变。
所以答案C比A好，而不是因为threshold和oil price没有关系。

-- by 会员 wyw1018 (2012/9/2 13:29:11)

虽然oil price 下降原因是单一的 比如可能 oil-fired plants efficiency上去了 还是可以change threshold
那为什么不能假设  oil fired power plants efficiency上去了  但是油价上的更猛  所以threshold 也受了影响呢？

shirryzhan

研究了此题，也参看了其它论坛一些解释，觉得都没有解释到位或者基本都错误，虽然答案正确，当然很多都是参考了正确答案然后再去反推的。
并不是threshold和oil price没有关系，相反是肯定有关系的。只是答案A只给出了适用油价下降这一单一原因，而答案C可以适用解释油价上涨，下降和不变的所有情形都可以使thrshold economic viability维持不变。
所以答案C比A好，而不是因为threshold和oil price没有关系。

-- by 会员 wyw1018 (2012/9/2 13:29:11)

  我突然理解了  是不是C. Technological changes have increased the efficiency of oil-fired power plants里的efficiency已经包含了oil plants整体 more economical的意思 所以应该不用再考虑是否受原材料价钱的影响。。？