Wisdom is to the mind what health is to the body. what起什么作用?谢谢帮助

知识之于精神,一如健康之于肉体.what作连接代词时还有一种特殊的用法,用来比较说明两种事物相同或相似的关系,起比喻作用,相当于连接词as,意为“正如”、“好比”、“就象…一样”.现具体说明如下：
1．A is Bwhat C is to D．
1） Water isto fish what air is to man（水对鱼的关系就象空气对人的关系一样.）
2）Ten is tofive what eight is to four．（10比5等于8比4）
2．What C isto D,A is to B．
l）What foodis to the body,a book is to themind.（书籍对于心灵犹如食物对于身体.）
2）Whatthegun is to a fighter,the pen is to awriter．（作家的笔犹如战士的枪.）
3．What C isto D,that A is to B．
1）Whatnourishing food is to the body,that a good bookis to the mind．（好书对于心灵正如营养食物对于身体.）
2）What lungsare to the body,that parks are to thecity．（公园对于城市犹如肺对于人体.）