The book was written in 1946, the education system has witnessed great charges.
A. when
B. during which
C. since then
D. since when
网友回答
A. when
B. during which
C. since then
(答案→)D. since when
解析：since when中的when指1946，since when指的是since 1946，即指“自1946年起”。注意不要误选C，如果将C改为and since then，则可选C。