When I entered the room,I saw him ( ) in a front seat staring

When I entered the room,I saw him ( ) in a front seat staring at a picture in a strange way.
A、to sit
B、is sitting
C、sat
D、sitting
【正确答案】：D
【题目解析】：see sb. doing，句子含义为当我走进房间时，看见他以一种很奇怪的方式盯着那幅图看。