用括号内所给的适当形式填空。 1. You are ____ (suppose) to keep silent when the teacher is speaking in class. 2. When I was introduced to the American professor, we __ _ (shake) hands with each other. 3. People_______ (drop) by each other's house on the Spring Festival. 4. I feel very _______ (relax) when I listen to my favorite music. 5. They usually make plans __ (see) friends.