Mr Black goes into his usual teashop one morning, and sits in one of the seats at the counter . Many other people also come in, but none of them stays long. About fifteen minutes later, a young man and a young woman come in. There are only two empty seats at the counter, one on Mr Black's left, and the other on his right. The woman sits on one seat, and the young man sits on the other, but Mr Black immediately  asks to change seats with the young man so that he and the young woman can be together. "Oh, that isn't necessary," the young man says, but Mr Black insists . When the young man and the young woman are side by side, he says to her, "Well, this old man is very kind. He wants us to sit together. May I introduce myself? My name is John. What's your name?" The young man and the young woman are   _  .
friends a couple brother and sister strangers