Some boys were seen ————(play)football on the playground yesterday

问题描述:

Some boys were seen ————(play)football on the playground yesterday

可以填 :playing
意思是:昨天一些男孩被看见正在操场上踢足球.
或者 填:to play
意思是:昨天一些男孩被看见在操场上踢足球(指被看见踢足球的全过程).