by end of last term 和last term 什么区别呢?

题目

by end of last term 和last term 什么区别呢?
She must have studied very hard "by end of last term",hadn't she ?
She must have studied very hard last term,didn't she ?

答案

by the end of +过去某个时间,用hadn't
过去某个时间,用didn't