Tom ate more food ___was good for his health.为什么要填than?

题目

Tom ate more food ___was good for his health.为什么要填than?
句子没有问题的~是定语从句还是比较状语从句？用than的话，从句不是就没有主语了吗？

答案

more...than 比……多
汤姆吃了比对他的健康有益的更多的食物.
就是说汤姆吃得太多了
主语省略了,应该是
Tom ate more food than (the amount of food which) was good for his health