完成对话
A: How was your vacation, Fred?
B: ____ .
A: ____ ?
B: I went to the U.S. with my parents.
A: Cool! ____  in the U.S.?
B: First we went to Los Angeles(洛杉矶). My uncle lives there. Then we went to New York to see my cousin.
A: ____ ?
B: It was a little warm but sunny.
A: Did you see any movie stars in Los Angeles?
B: No, but I saw Tom Cruise in New York. He was at a restaurant! I got his autograph.
A: Wow! That’s great!  What else did you do in New York?
B: Hmm…Oh yeah, I also went to Central Park.____ there! .

生活中的下列变化，一定发生了化学变化的是（　　）

集合 A={1，2，3，4，5}，B={1，2，3}，C={z|z=xy，x∈A且y∈B}，则集合C中的元素个数为（　　）