A group of students at Sunshine School are holding a charity sale to raise money to help their classmate Lisa. Lisa had cerebral palsy (大脑性瘫痪) when she was a baby. People who have cerebral palsy have trouble walking. Although Lisa has to use a wheelchair (轮椅) to help her get around, she never gives up.

    "Lisa has the chance of standing up, but she needs an operation (手术)," Lisa's doctor said.

    "I never thought that I could stand up one day. Actually the news is surprising for me. However, when I see my parents are worried about the cost of the operation, I am sad. I don't mind if I can't stand up," Lisa said. "Now my classmates and some other kind people are helping me. I am really thankful."

    "While helping Lisa, we can also learn a lot. We have learned how to cook cookies (曲奇饼) and how to sell them," Betty, one of Lisa's classmates said. "Now we know making money is not easy, so we won't waste money anymore."

    Alex, a boy in Lisa's class, told a reporter that they could raise about $100 every day. "We have raised about $2,100 so far. We hope we will raise more money for Lisa's operation," he said.

____是黄种人集中地地区之一。 ____是白种人集中地地区之一。

盒子里有80枚白子和50枚黑子，每次取走3枚白子，同时放入3枚黑子，像这样取放(    )次后，白子和黑子的枚数正好相等。