主要是修饰Enery这个词的一致性要求.简单地说,这里比较的是：
the energy expended in traveling from the Earth to the Moon
vs.
the energy expended in traveling from the Moon to the Earth.
你可以尽量把后者(the energy expended in traveling from the Moon to the Earth),但是不能简化到“断章取义”的程度,你就会发现E才是对的了.