OG17 的一道强调句的题

二零一八

While it costs about the same to run nuclear plants as other types of power plants, it is the fixed costs that stem form building nuclear plants that makes it more expensive for them to generate electricity.

正确选项：While the cost of running nuclear plants is about the same for other types of power plants, the fixed costs that stem from building nuclear plants make the electricity they generate more expensive.

这是整个题干和正确选项，其他选项我就不打了
OG 解释说it is X that does Y 的强调句型必须是知道这个动作的doer是谁，然而这道题不能justified， 所以不能用 it is the fixed costs that难道不是动作的发出者是fixed costs吗 fixed costs 阻止了建立原子能的工厂巴拉巴拉的
求解为什么？？