The missing boy was last seen ___ near the river.A.playing　 B.to be playing　　C.play　　D.to play为什么选A?see的被动语态不用还原to吗?为什么不用B,

see sb do sth改成被动语态为 be seen to do sth， see sb doing sth 改成被动语态为be see doing sth 被看到正在河边玩耍

因为 “seen”被看到 是被看到正在做某事。
关于B选项，to be playing 是“正在被玩”，这显然不对

你的问题很简单
1 是由主动语态变来的：Someone saw the missing boy playing near the river.
see sb.doing:看见某人在做某事
see sb.do sth看见某人做了某事
2 因为someone看见他的时候,the boy正在玩,所以就成了saw him playing
3 再把Someone saw the missing boy playing near the river.改写成被动语态：
The missing boy was last seen playing near the river.改写成被动语态时,把原来的宾语变成主语,在把saw改写成was seen,后面的playing不变.现在,你明白了吗?