请问是on the playground还是in the playground

问题描述:

请问是on the playground还是in the playground

以下文字摘自《英语基础词汇用法词典》
……表示“在运动场(上)”“在操场(上)”playground前既可用in,也可用on,但以用in为普通:They are playing baseball in [on] the playground.我的同学在运动场上打棒球.