The school held a table tennis selection competition, and each player had to compete with all the other players. A total of 21 games were played. How many people participated in the selection competition?
学校举行兵乓球选拔赛，每个参赛选手都要和其他所有选手各赛一场，一共进行了21场比赛，有多少人参加了选拔赛．
<result>7</result>