Question

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,D？

Answer 1

你好。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？

Answer 2

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

Answer 3

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