答案：A
解释：这里是及物动词hide的过去分词，表示句子的主语I（我）是过去分词hidden的承受者。非谓语形式。完整的句子是：As I was hidden behind the door,I wasn't found by the fellow.
翻译：由于我藏在门后面，我没有被那家伙发现。
提示：过去分词作状语，句子的主语是分词动作的承受者；现在分词作状语，句子的主语是分词动作的执行者。
例如：Waiting for a bus,I saw Jack.句子的主语I(我)是现在分词waiting的执行者。“等公交车的时候，我看见了杰克。”这个句子等于“While I was waiting for a bus,I saw Jack.”
学习要点：英语中这几个词必须弄清楚：seat,hide,lose,dress,他们的句型是：seat oneself ,hide oneself ,lose oneself,dress oneself.
帮助理解：I seatd myself.=I was seated. I hid myself behide the door.=I was hidden behind the door. I lost myself in the forest.=I was lost in the forest. I dressd myself in red.=I was dressed in red.
延伸理解：As she was dressed in red,she could easily be found.=Dressed in red,she could easily be found.句子的主语she是dress动作的承受者。As she was dressed in red,=As she dressd herself in red,。