题目

阅读理解 Our school is on Guangming Street. It's next to the People's Library. There is a park across from our school. The pay phone is between our school and a post office. Our school is very beautiful. There are many trees and flowers in it. If you want to come to our school from the airport, take a taxi and go down Center Avenue. You can see a big restaurant on your right. There is a bridge near the restaurant. Go across it and turn left. Then go down Bridge Street. Turn left at New Park and go down Guangming Street. Our school is across from a bank. （1） Our school isthe Peop设α，β为两个不重合的平面，l，m，n为两两不重合的直线，给出下列四个命题：①若α∥β，l?α，则l∥β；  ②若m?α，n?α，m∥β，n∥β，则α∥β；  ③若l∥α，l⊥β，则α⊥β；  ④若m、n是异面直线，m∥α，n∥α，且l⊥m，l⊥n，则l⊥α．其中真命题的序号是    ．