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.

老师在课堂上做了下列实验(实验所需器材如图所示)：把一个生鸡蛋放入一杯清水中，鸡蛋沉入杯底，往清水中溶入少量食盐，鸡蛋仍沉在杯底，继续溶入食盐，鸡蛋慢慢浮上水面。根据这个实验事实，请你回答下列问题：

快乐阅读。

       我今年87岁，今天回顾过去，说不上_________，也谈不上_________，我只是老老实实、平平凡凡地走过了这一生。我思索，我追求，我终于明白生命的意义在于_________而不于_________。我在回答和平街小学同学们的信中说：“我愿意再活一次，重新学习，重新工作，让我的生命开花结果。”

根据图中通电螺线管的N、S极，在图中标出磁感线的方向和电流方向．