下列说法正确的是（　　）

已知|AB|＝2，动点P满足|PA|＝2|PB|，试建立恰当的直角坐标系，动点P的轨迹方程为____．

问题一：食盐加碘可以有效地预防甲亢

问题二：食盐是比较稳定的，而说明中强调“防热”是否因为受热会造成碘的损失？

    Christina is an American girl. She is a good student. Every morning she gets up at 6:00. She has her breakfast at home. Then she wears her uniform and goes to school. She studies hard every day. She eats in the dining room when she is in school. After school, she does her homework first, then she cleans the room and helps her mother with the dinner. Before she goes to bed, she usually reads some books. Then at 10:00 p. m. she goes to bed. She thinks she is happy every day. What do you think of her?

—It's on the desk.