the missing boys were last seen 空格 near the river; a:playing ; b:to be playing,

题目

the missing boys were last seen 空格 near the river; a:playing ; b:to be playing,
我这儿的答案是b,

答案

选择A
理由 see somebody doing 是固定结构,无论see用的主动形式还是被动形式都用doing；
答案可能依据的是see sb do由主动变被动时要加上不定式符号to.
供参考.