Hello! I'm Wang Lin. I like sports very much. Basketball is 1 favourite sport. Tom is my friend. He is an 2  boy. He is from America. He is 12 3  old. He likes sports, too. He has（有）a big ball collection（收藏物） 4  his room. There are 5  balls in the room. Tom's  6  aren't at home.  7  his sister Dee is at home. She is in the room now. Her favourite 8  is football. I don't like  9  because（因为）she isn't nice to me. I want（想要）to have a 10 collection too. Then I can have many balls. Do you want to have one?

物体之间都存在一种相互的引力，人们称这种引力为____。