用mustn't 或must完成句子。 1.Cars, buses and bikes ______ go when the traffic light is red. 2. The flowers______be watered every day, or it'll die. 3. No matter who wants to come in,he ______ knock on the door first. 4. You must speak to the old politely, ______you? 5. The children ______swim in the lake. It's not safe. 6. I haven't seen Alice several years. She ______be a big girl.