Mr. Brown works on a farm. Mr. Green works in a park. But they don't know each other. Today is Sunday. Mr. Brown comes to the park. He feels tired. He wants to have a little rest. He sees a red chair under a tree over there. So he walks to the chair. Mr. Green is behind him. Mr. Green runs to the chair with a board when he sees Mr. Brown come to the chair. But Mr. Brown runs faster（更快） than Mr. Green. Then he sits on the chair. Mr. Brown is very happy. But Mr. Green doesn't say anything. He gives the board（板子） in his hand to Mr. Brown. It says, "Wet Paint(油漆未干). "

如图，直线a∥b，直线c是截线，如果∠1=50°，那么∠2等于（　　）

人与人之间的平等集中体现在（   ）