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    Hi, boys and girls! My name is Gina. I'm an assistant at Mr. Miller's Store. Now let me say something about the store. On the first floor (层), we sell clothes for girls. The skirts in white, red, black and yellow are only $35. We have socks in all colors. They're only $8 for two pairs. How much are our sweaters? Only $25!

    If you are a boy, you can go to the second floor. Look at those baseball hats. Do you like them? They are only $6. Are you very fat? We have some big jackets for you. They are $40. We have some shorts, too. They are only $18.

    What do you need? Come to Mr. Miller's Store now!

在平面直角坐标系中，一次函数y=ax+b（a≠0）的图形与反比例函数y= $\frac{k}{x}$ （k≠0）的图象交于第二、四象限内的A、B两点，与y轴交于C点，过点A作AH⊥y轴，垂足为H，OH=3，tan∠AOH= $\frac{4}{3}$ ，点B的坐标为（m，﹣2）．

《汉书》记载“江南地广或火耕水耨……不忧冻饿，亦亡（亡：无）千金之家。”而据《宋书》记载南朝时期江南已经“地广野丰”，“丝绵布帛之饶，覆衣天下”。促成这种变化的最主要原因是（    ）