逻辑问题请教。新PREP上滴~~

yangyanghaoren

Technological improvements and reduced equipment costs have made converting solar energy directly into electricity far more cost-efficient in the last decade.However, thethreshold 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.

答案为C，说句实话，本人真心没明白这题到底什么意思..呜呜..望大家帮忙解答，谢谢了

yangyanghaoren

大家帮忙啊，自己顶~~~感激不尽

wyw1018

首先定义这个threshold：
假如发一度电：老太阳能设备的成本是$M，老燃油发电需要耗油N barrel，油的成本是每barrel $P，现在的情况是M=N（P+Q），Q即是这个threshold,可得出Q=（M/N）-P，M下降，要保持Q不变，可以有P下降，也可N下降来满足。具体下数据假设M=200，N=2，P=65，Q=35，200=2（65+35）。
现在太阳能设备效率上升，假设发一度电新太阳能成本降低为150，而新燃油发电设备N，Q不变，则P可下降为40，150=2（40+35），因此A也有可能正确；再假设新燃油发电设备N下降为1.5，即使P 不变，也会保持Q不变，即150=1.5（65+35）；再假设即使P上升为85，也可因为N下降为1.25满足，150=1.25（85+35）；再假如P下降到55，也可因N下降为1.67满足，150=1.67（55+35）。因此C中无论油价上涨，下跌或不跌都可由N下降（即效率提高）来解释，优于A中只有油价下跌这一单一原因解释。
也就是说A并没错，但是不如C好，注意most to help explain这most 这个词。