题目

 --- Which city do you want to_______?  ---I want to go to Beijing.            A. go        B. to go to         C. go to      D. going 答案： B        如图，在△ABC中，点D，E，F分别是△ABC的边AB，BC，AC上的点，且DE∥AC，EF∥AB，要使四边形ADEF是正方形，还需添加条件：__________________．