We saw many students playing on the playground, most of them girls.
A. are
B. being
C. were
D. have been
网友回答
A. are
(答案→)B. being
C. were
D. have been
解析：考查独立主格结构。句意：我们看到很多学生在操场上玩，他们中大部分是女生。most of them girls.前缺少连词，可知下文为独立主格结构。现在分词短语表示正在进行的伴随性动作，结合语境可知选B。