询问过去状态：What were you doing? — 英语选择题
题目
昨天下午4:00，Lily在为Amy布置生日会场；4:30，派对开始了，Lily在吹气球。Amy的妈妈当时不在场，回家后想知道4:30时Lily在做什么，她应该怎么问Lily才最合适？ A. What were you doing at 4:30 yesterday? B. What are you doing now? C. What were you doing at 4:00 yesterday? D. What did you do at 4:30 yesterday?