— Must I be in hospital for a week, Doctor? 

— No, you           . You can go back home tomorrow. [ ]

A. mustn't

B. needn't

C. must

局部通风机停止运转且未恢复正常通风之前，停风区域中甲烷的浓度达到3%时闭锁装置应能闭锁局部通风机的电源。