The book was written in 2001,___the education system has witnessed great changes.A.in which B.since when 为什么不选A?

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The book was written in 2001,___the education system has witnessed great changes.A.in which B.since when 为什么不选A?
The book was written in 2001,___the education system has witnessed great changes.
A.in which B.since when 为什么不选A?

The book was written in 2001,___the education system has witnessed great changes.A.in which B.since when 为什么不选A?
根据后面的HAS WITNESSED (现在完成时)可知 是用SINCCE WHEN,如果用A是一般过去时.
加油!不明白再问!

since then 与现在完成时连用