WhenIentered,theyweretalkingabouttheteachersandschools　...

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When I entered, they were talking about the teachers and schools　　　　they had visited.

A. which B. whom C. that D. who

C。当先行词既包含人又包含物时, 关系代词只能用that, 因为that可指人, 也可指物。