It's very kind of you _____(say) so.并说明理由,

It's very kind of you to say so.
be kind of somebody 后要接to do something.这是一种固定用法.
例句：
It's verykindof you to help me.
谢谢您好心帮助我.
It's very kind of you to tell me.
你来告诉我真的太好了.
It's very kind of you to help me with these problems.
你帮我解决了这么多难题,真是太好了.