摘要：1．All passengers are expected to fasten the seat belts when the plane takes off or lands. A．on the board B．on board C．on boards D．on a board 解析:选B.on board＝aboard上船/飞机/车等,在车/船/飞机上等.
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All passengers________are expected to fasten the seat belts when the plane takes off or lands.
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A. on the board
B. on board
C. on boards
D. on a board
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B. on board
C. on boards
D. on a board