有关形容词比较级修饰名词：

Hydrogen, which is ______ than oil, is pretty easy to produce... 

答案是d.a much more abundant energy source, 但我的问题是选项b中 an energy source more abundant 为什么不对，形容词比较级做后置定语修饰名词不是相当于省略的定语从句吗？.. is an enrgy source which is more abundant than oil