All passengers        are expected to fasten the seatbelts when the plane takes off and lands.

A. on the board   B. on board      C. on a board    D. on boards

All passengers        are expected to fasten the seatbelts when the plane takes off and lands.

A. on the board      B. on board    C. on a board  D. on boards

All passengers 　　   are expected to fasten the seatbelts when the plane takes off and lands.

A. on the board　　　　　　 B.on board?　　    C.on a board　　　　  D.on boards

A. on the board　　　　　　 B.on board?　　    C.on a board　　　　  D.on boards