Maybe this is a stupid question, but in Minna no Nihongo it says to omit だ when giving a positive answer to a question.
But it is not explained if one can omit だ when answering this way:
Answer: ううん、ちゅうごくじん (だ?)。
Answer: にほんご (だ?)。
Do I have to put だ at the end of these two answers or should I omit it?