两条直线相交，如果其中一个角是直角，那么其它三个角都是____角，这两条直线____．

    A bus arrived and an old woman got on it slowly. She wore no shoes.

    A little boy pointed at (指) the old woman and said to his mother, “Look, Mum. She isn't wearing shoes.” “Don't point at others, Tom. It's rude,” the mother said and then looked out of the window.

    “She must have children,” said a woman in a fur coat. “Her children should buy shoes for her.”

    “People should learn to save money,” said a well-dressed man. “If she saved money when she was young, she wouldn't be so poor now.”

    Then all the passengers (乘客) on the bus stopped talking.

    At the next stop, a young man got on the bus. He was listening to music. When he saw the old woman, he stopped moving. He carried his eyes from her feet to his own. He was wearing his expensive new trainers. He saved money for months to buy them. Unexpectedly, the young man took off his trainers and socks. He stood before the old woman. “Madam,” he said, “I see you have no shoes. Well, I have.” Carefully and gently, he put his socks and fine trainers on the old woman's feet.

    Just then the bus arrived at the next stop. The young man got off the bus and walked away, wearing no shoes in the snow.

    “Who is he?” one asked. “He must be an angel (天使),” said another. But the honest (诚实的) little boy said, “No. He was only a man.”

独立战争和南北战争是美国历史发展的两个重要里程碑。二者的相同点主要是（     ）