It was announced that only when the fire was under control ____to return to their homes.   [ ]

A. the residents would be permitted  

B. had the residents been permitted  

C. would the residents be permitted  

D. the residents had been permitted

下列说法正确的是()

A．自由水的自然运动频率高

B．结合水依附在大分子上，其自然运动频率低

C．自由水运动频率明显高于拉摩尔共振频率

D．结合水运动频率介于自由水与较大分子之间

E．以上均对