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”。
