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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

The book was written in 1946，　　　　 the education system has witnessed great changes.?

A.when　　　　　　　 B.during which?　　 C.since then　　 D.since when

(  )24. The book was written in 1946， 　　　　 the education system has witnessed great changes.

A. when              B. during which       C. since then   D. since when