The Boeing 747-400 airplane has reached its cruising altitude of 10,000 meters and cruising speed of 900 km per hour on its way from London to Beijing. The Purser has announced that the passengers are now free to unfasten their seat belts and move around the cabin; however passengers should keep their seat belt fastened when seated. The passenger cabin layout consists of three sections. The 747-400 is unique in having two decks; on the upper deck, behind the flight crew there is the first class section. On the lower deck, at the front, the business class section, for passengers who pay the full fare: while behind is the much larger economy section for passengers who have special cheap rate tickets. The stewards and stewardesses—14 in all—are now busy around the galleys. A choice of drinks, followed by dinner, video entertainment including a film and finally breakfast in the morning is the order of service. Individual headphones are distributed, and these, when plugged in to the arm of the seat—give a variety of in-flight musical programs. Also in the arm of the seat are the controls for a personal reading light, fresh air control and call button. Dinner is served on specially prepared trays, perhaps prawn cocktail, followed by a choice of roast chicken or braised steak with chocolate mousse to finish. The chicken and beef has been heated in the special microwave ovens in the galleys. It is hard to believe that a hot, three-course meal can be served while the plane is flying at this altitude and speed. Dinner over, the cabin lights are dimmed, the blinds by the windows are drawn, the screens unfold and the film begins. Now, for the first time the cabin crew take it in turns to have a rest while the passengers watch the film or snooze dreaming of arriving in Beijing in the morning.3. During the flight, the cabin attendants should do the following except （ ）.