同学你好，该知识点来自沪江网校《新概念英语1、2、3、4册连读【周年庆特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
这里to表示“对于……”，be known to…… 强调对……来说是已知的，比如
It must be stated that this illness is one of the most complex conditions known to man.
必须声明，这种疾病是人类已知的最为复杂的疾病之一。
He wants to keep them all in his office for reasons best known to himself.
他想把他们都留在他的办公室里，原因只有他自己知道。

祝同学学习进步~