The book was written in 1946, the education system has witnessed great changes.
A. since then
B. when
C. since when
D. during which
网友回答
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。