[求助] OG-153

OG-153：

Tech 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?

我的问题是：

为什么提问部分不是“Which of the following, if true, does most to help explain why the increased cost-efficiency of solar power has not increased its threshold of economic viability?”

题干的括号部分给出了the threshold of economic viability for solar power的解释，很显然是根据油的升价来定义的。既然太阳能more cost-efficient, 也即power plants more economical, 那么应该increase its threshold of economic viability才对呀。我觉得自己都糊涂了。

请各位考友赐教！ 谢谢。