I heard some children __(sing) a song just now .

问题描述:

I heard some children __(sing) a song just now .

singing,我听见小孩子刚刚正在唱歌,hear sb doing/do sth,听见某人正在做什么

后面是just now,所以是singing

填singing
hear sb doing sth听到某人正在做某事
just now刚刚
刚刚我听到有孩子在唱歌