单选题 He isn't legally responsible for his nephew, but he feels he has a moral ______ to help him.
【正确答案】 B
【答案解析】(obligation义务)。句意为：从法律上讲，他可以不对侄儿负责，但他觉得他在道德上有义务帮助他。A．observation观察；C．objection反对；D．obstruction阻塞、堵塞。