请教逻辑OG-153

153. 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.
答案是C,我选的是A,我的分析:扩号中已经明确对threshold进行
了说明,threshold由必须增加的price决定.如果solar energy变的
far more cost-efficient,那么如果oil方面没有变化的话,threshold
变小了(也就是oil需要增加的价格减少了),这样,oil价格减少的话
就可以保持threshold不变了.我这样分析有没有问题?C也有道理,
不过关系不够直接,还要由efficiency of oil-fired power plants,推
价格的关系,我觉得还不如A好.但答案是C,为什么?OG的解释是:
(Actual oil prices control how far, given the viability threshold,
solar power is from economic viability but do not figure in the
determination of the threshold, so choices A and E are incorrect.)
我不太明白.