结构推理 One evening my sister was sleeping in her bedroom upstairs while her eight-year-old son, Jason, was in his room on the first floor. Suddenly she was wakened by a loud crash. A van has run off the road, smacked into the side of the house and come to a stop in the living room. Her first thought was of her son.
   “Jason!” she yelled out—and from downstairs her son yelled back, “I didn't do it, Ma!”