I was walking along a busy road when I noticed a young man standing near the crossing selling toys.1 I would hurry past such sellers. But I found this young man was 2 so I stood for some time without crossing the street. I wanted to see how he sold his things.

    Soon, a young mother with a little girl came by and the child set 3eyes on one of teddy bears. The mother asked for the4 and it was only one dollar. She 5 out a 10-dollar note from her handbag and told the young man that she was giving him 10 dollars and wanted the6 back. I was wondering how he was going to do it. He told the young mother to put the money in the bag 7 around his neck and take out her change.

    I realized that this young man depended on believing people not to cheat him. There was no way for him to know 8 someone would take out more than they should have. I was so 9 I bought a car model from him that I did not need. As I walked down the street, I gave the car model to a kid walking with his father.

    I kept thinking that the world would be a better place if we could all learn to 10 others more, like the young blind street seller.

  20世纪初，著名的物理学家爱因斯坦建立了____，使人们对时空观的认识产生了根本性的变革.

一个正比例函数的图象经过点（2，﹣4），则这个正比例函数的表达式是____