degrees做先行词的定语从句that为何省略

as debt-laden students head for degrees they think will maximize their earning potential, many vital courses suffer.

请问这句话degrees做先行词的定语从句，that为何省略，做宾语了吗？

As debt-laden students head for degrees they think will maximize their earning potential, many vital courses suffer.

they think will maximize their earning potential 这是嵌入式定语从句。定语从句本身是个主从复合句，they think是主句，that will maximize their earning potential为宾语从句，关系代词that并不在主句中作成分，而是在宾语从句中作主语。关系词所在的分句，嵌入到they think这个分句中作宾语，这种定语从句就叫做嵌入式定语从句 embedded relative clause。关系代词在定语从句中作主语一般是不能省略的。但假如关系代词在嵌入式定语从句中作宾语从句的主语，则是可以省略的，而且是非常容易见到的。

再给你一个嵌入式定语从句省略作主语的关系代词的例子：

This is the book my teacher says is very useful. book后省略了that。

网友可以在答疑网搜索嵌入式定语从句“。