Someone from João Pessoa, Paraíba, BR posted a whisper, which reads "Today I presented a seminar at the university and I started with "Hello". The audience replied with "It's me". I've never felt so comfortable speaking in public in my life.
Person C knows his had is black, because if it was white, Person D would have seen two white hats and said his color ;D The best riddle I've ever heard. (I'll post answer in comment section if everyone gives up).