摘要：9．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
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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