Question

关于定语从句的一道英语题

the book was written in 1946,____the education system has witnessed great changes

答案是since when,可是不是the education system has witnessed great changes since 1946 对1946的定语从句，不是用which吗？？

本人小菜，请教指点，谢谢！！~要解释哦~
我是说since which
不是which哦~

修饰时间就一定要用when 吗？1946不是表示年吗，用which
若是in 1946才是用when吧！

Answer 1

in 1946说明是时间状语，当然用when/in which了,the education system has witnessed great changes since 1946 对in 1946的定语从句……
不懂再问我