the front door he had to enter the room through the back door.A. Seen; paintedB. Seeing

问题补充：

the front door , he had to enter the room through the back door.

A. Seen; painted

B. Seeing; painted

C. Being seen; being painted

D. Seeing; being painted

答案：

A. Seen; painted

B. Seeing; painted

C. Being seen; being painted

(答案→)D. Seeing; being painted

解析：see 表示的动作与句子谓语动词（had to enter）所表示的动作几乎同时发生，且与其逻辑主语（he）为主动关系，故用一般式。由句意“看到前门正在被漆，……”可知，第二空应用being painted。