Question

一个史上很难回答的英语问题---务必高手进---在线等

35、The book was written in 1946,_____the education system has witnessed great charges.
A：when B：during which C：since then D：since when
答案是D，我想知道
1、这个句子是非限定性定语从句吗？
since在此句中不是介词吗？since是介词的话，后面的从句应该是名词性从句才对啊。
2、when是代替1946，还是in 1946，如果是代替1946，那么when不就是关系代词了吗？
3、是不是此句中还有别的语法我没有注意到。
请高手帮忙。

Answer 1

你首先看到 has witnessed 是现在完成时，肯定要加表示时间延续的短语，这里就是since when，这里的when 指代The book was written in 1946。你可以换下顺序The education system has witnessed great changes since when the book was written in 1946。简略些就是The education system has witnessed great changes since 1946.

Answer 2

是非限 When指1946 即从1946起 Since是介词 从那以后