在调查社区的生物种类时，要仔细观察、认真记录，正确的做法是（  ）

水深2米处水的压强为　____ 　帕，重为10牛，体积为5×10^﹣4m³的石块浸没在水中受到的浮力大小为　____ 　牛，它受到重力与浮力的合力大小为　____ 　牛．

    Hi. My name is Betty. I'm from Canada. I'm a student. I'm in Class Two, Grade Five. There are four people in my family. They are my father, my mother, my brother and me. My father is an English teacher. He works at a school. My mother is a doctor. She works in a hospital. My brother is a student, too. He is in Class Two, Grade Three. I love my family.

①说说看，你是怎么知道各种活动完成的时刻的？

②拿出钟来拨拨看，你做对了吗？