-May I go to the cinema,mum?Certainly.But you--be back by 11

选 C must 根据语境,应该用must 必须
May I go to the cinema,mum?
翻译：
妈妈,我可以去看电影吗?
Certainly,but you must be back by 11 o'clock.
翻译：
当然,但是你必须十一点前回来.