问题补充:
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:
A. when
B. during which
C. since then
(答案→)D. since when
解析:since when引导定语从句。因为句中的谓语has witnessed是现在完成时,所以要用since when。when指代上文中的in 1946,表示“这本书是1946年写的,从那以后,教育体制发生了巨大变化”。