Opens choice or happens choice

Hello everyone,

I watched a comedy film, in which the teacher wrote some subjects on the board for his students so that each student would pick a subject to talk about. All the students chose their subjects to talk about and they did. One student was left and one subject was left on the board. When the teacher asked the remaing student to stand up, the student stood up and said I will pick a subject, but the teacher laughed and said only one subject is left so it is "opens choice". Here, I'm not sure if he said "happens choice" or "opens choice". Could you please tell me which one is correct or possible in that situation? And what does it mean?

Regards,

JA