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Attention, please! All the mobile phones ______ during the meeting.

A. must keep off           B. have kept off     C. are keeping off   D. must be kept off

Attention, please! All the mobile phones  ______ during the meeting.

A. must keep off   B. have kept off     C. are keeping off      D. must be kept off

Attention，please! All the mobile phones _____________during the meeting．

A．must keep off                                   B．have to keep off

C．a(chǎn)re keeping off                                   D．must be kept off