填空。
①You mustn't walk around during landing. ____ A. On a bus
②You mustn't talk to the driver.____ B. On a train
③You must wear a helmet .____ C. On a bike
④You must wear a seatbelt.____ D. On a car
⑤You mustn't put your head out of the window.____ E. On a plane
⑥You must wear a lifejacket.____ F. On a boat
两实数根，则
等于（ ）