问题补充:
The book was written in 1946, the education system has witnessed great changes.
A. since then
B. when
C. since when
D. during which
答案:
A. since then
B. when
(答案→)C. since when
D. during which
解析:句意:那本书被写于1946年,自从那时以来教育体制见证了伟大的变化。因为逗号前后都是句子所以必须用连词,故排除A;这里是非限制性定语从句,先行词是1946,又根据谓语动词has witnessed可知,since与现在完成时连用,,故选C。