题目

---Where is mother?    ---She is in the kitchen. She ___ the dinner for the party the whole afternoon.    A. is preparing         B. was preparing           C. has preparing            D. has been preparing 答案：D 解析: 略19．入射光线与平面镜的夹角由90°减小到60°时，反射角（　　）A．由90°减小到60B．由30°增大到60°C．由60°减小到30°D．由0°增大到30°