:----- the front door -----,he had to enter the room through

:_____ the front door _____,he had to enter the room through the back door.
选项:a、Seen/painted b、 Seeing/ painted c、 Being seen/ being painted
d、 Seeing/being painted
选哪个?为什么?请分析并翻译整句
这道题是选D还是选B啊,