When I was a very young children,my father created a regular practice I remember well years late.Every time he arrived home at end of the day ,we’d greet het at the door.He would ask who we was and pretend not to knowing us,Then he and my mother would have had a drink while she prepared dinner and they would talk about his day and hers.While they chat,my father would lift my sister and me up to sit in the top of the fridge.It was both excited and frightening to be up there！

My sister and I thought he was so cool for putting us there.

3（）

什么是离子交换树脂的选择性？