many books____(send)to school since then为什么不写have been sent,而写had been sent

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many books____(send)to school since then为什么不写have been sent,而写had been sent
many books____(send)to school since then为什么不写have been sent,而写had been sent

many books____(send)to school since then为什么不写have been sent,而写had been sent
since then 表示的是在过去的过去 要用过去完成时