It's very kind ( )you to say so.A.to B.of C.for

B
be kind of
如：she is very kind to me 和 It's very kind of him to help me 。
一个用to 一个用of 。
解释:to me,表示kind的对象是me,也就是说 她对我和善!
of him 表示的是kind的主语是him,意思强调的是 对我友好帮助的人是 他!