题目

 逆城市化现象的原因主要是（    ） A. 郊区有更多的就业机会       B. 市区环境恶化        C. 城郊交通不便                 D. 市区环境得到明显的改善 答案：BI _________ my homework last night when I heard a loud knock on the door. [     ]A. did B. was doing C. have done D. had done