- What do you know about kite surfing?

- It is _________ exciting water sport.

平面内到两定点F₁（﹣3，0）、F₂（3，0）的距离之差的绝对值等于4的点M的轨迹（　　）

   Bob was happy. He was at a new school, and the other students were friendly. “Hi, Bob!” They said. But some students said, “Hi, Peter!” Bob didn’t understand. He asked another student, “ Why do some students call me Peter?”
   “Oh, that is easy to answer,” the student said. “Peter was a student here last year. Now, he goes to a different school. You like Peter. Some students think that you are Peter.”
   Bob wanted to meet Peter. He got Peter’s address(地址) from a student and went to Peter’s house. Peter opened the door. Bob couldn’t believe his   eyes. He looked really like Peter! Bob and Peter had the same color eyes and the same smile. They had the same black hair. They also had the same birthday.
   Bob and Peter found out that they were twin brothers. They both were adopted(领养) by two different families. Soon after the boys born, one family adopted Bob, and another family adopted Peter. Bob’s family never knew about Peter, and Peter’s family never knew about Bob, either.