家规表达与原因 — 英语选择题

小明家的家规是：晚上九点必须睡觉，因为明天要早起。下面哪句话表达正确？ A. I must go to bed at nine, because must get up early tomorrow. B. I must go to bed at nine, because I must get up early tomorrow. C. I must go to bed at nine, because I get up early tomorrow. D. I must go to bed at nine, because I must get up early.