It is --nice of you to say so

此处填副词,排除B,D
truly
ad.
1.真实地;如实地;真正地
He is now truly American.
他现在真正美国化了.
2.确切地;精确地;准确地
3.真诚地;忠实地
I am truly grateful for all your help.
我真诚感谢你的一切帮助.
4.非常,很
5.(常用作插入语)的确,真的
6.有理地;合法地
really
ad.
1.真地,确实,实际上
Did she really say that?
她真的那么说了?
2.很,十分,全然
It's really cold today.
今天很冷.
3.(用以加强语气)实在,其实
You ought really to have asked me first,you know.
要知道,你本应先问一问我的.
4.(表示疑问、惊讶、恼怒等)真的;是吗;真是的
Well,really!You needn't have said that.
咳,真是的!你当时实在没有必要那么说.
此处really更合适

我觉得应该选C

really为副词，用来修饰形容词nice