--- Listen! David is talking happily with his mother in the next room. --- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.
I don’t know _________.
The MP3 ______ too much. He ______ 200 yuan _______ it .
The art club is for members only. You can’t go in _______ you are a member.
She likes doing her homework _______ the radio _______.
In winter, we must pay attention to _________.