Question

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，！！！！

Answer 1

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。

Answer 2

答案错了
see sb. doing:看见某人在做某事
see sb. do sth看见某人做了某事
变被动后为Sb was seen doing (to do)不会出现to be doing

Answer 3

答案应该选a:playing

see后面接双宾语有三种用法，see sb do sth, see sb doing sth, see sth done sth。这句话的意思是小孩最后被人看到是在河边玩，应该是强调一种他正在玩的时候被看到的状态，因此选进行时playing。

Answer 4

playing

Answer 5

a

Answer 6

b