单选题 When I was sent to prison, I really felt I had ______ my parents ______.
A. let... off B. let... down C. let... out D. let... alone

【正确答案】 B
【答案解析】[解析] 各选项的意思为：let off“让下车；引发”；let down“使……失望”；let out“出租；使……出来”；let alone“更不用说”。根据句意可知，答案是B。