答案选C.动词不定式的一般式形式表示将要做的事,（医生竭尽全力）The doctor did what he could 就是为了去挽救那个在事故中严重受伤的女孩（to save the girl who was badly hurt in the accident）.
to save the girl who was badly hurt in the accident整体是动词不定式短语作状语,表示目的.
to save动词不定式；the girl是to save 的宾语；who was badly hurt in the accident是定语从句,修饰the girl.
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