to master some knowledge of first aid is(of/for/with/to)grea

这里的主语是动词不定式短语"to master some knowledge of first aid",谓语动词为系词"is",表语用了名词.主语与表语（抽象名词）之间不能等同,所以要用形容词来做表语.这里的"of"就起到了这样的作用.选"of".