The missing boy was last seen ___ near the river.

A

感官动词 + doing/to do
感官动词see, watch, observe, notice, look at,hear, listen to, smell, taste, feel + do表示动作的完整性,+doing表示动作的进行性.例如：
I saw him work in the garden yesterday.
昨天我看见他在花园里干活了.（强调"我看见了"这个事实）
I saw him working in the garden yesterday.
（强调"我见他正干活"这个动作）昨天我见他正在花园里干活.

典型例题 1）They knew her very well.They had seen her ___ up from childhood.　
A. grow　B. grew　C. was growing　D. to grow
答案：A.因题意为,他们看着她长大,因此强调的是成长的过程,而非正在长的动作,因此用see sb do sth 的句型.

2）The missing boy was last seen ___ near the river.　
A. playing　B. to be playing　C. play　D. to play
答案A. 本题强调其动作,正在河边玩,应此用see sb. doing sth句型.