51. D 【解析】细节理解题。根据第二段句子As I came near the window……found myself face to face with a man who had just walked into the room.可知当史密斯夫人检查房间的时候坏人进入了房子。故选D。
52. B【解析】细节理解题。根据第二段第三句I went into the kitchen, the storeroom, the living room, and finally the dining-room.可知史密斯夫人最后来到餐厅，绑架事件发生在这里。故选B。
53. A 【解析】细节理解。根据下句When I woke up, I found they had tied me to the chair, and a scarf round my mouth stopped me from making any sound.中的关键词woke up可知斯密斯夫人昏迷了。故选A。