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

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

since when以为“从那时起”
when时间副词,这里指代时间 1946,相当于since when的意思就是since the year 1946
自从1946年起,教育体系经历了巨大变革

首先排除AB 因为后面用的是has witnessed现在完成时态 所以用since when引导的从句