During the Olympic Games many people served as volunteers,(

During the Olympic Games many people served as volunteers,( ) were young college students .
A most of them B most of which C most of whom D most of what
但是我想问一下,这个先行词在从句中不是做主语吗?而WHOM是引导宾语的,为什么选C?如果选项有 most of who ,这个选项对吗?
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