如图，在正方体ABCD﹣A₁B₁C₁D₁中，E，F分别是棱AB、BC的中点，则平面A₁DE与平面C₁DF所成二面角的余弦值为（   ）

在设计对照组实验时要充分考虑变量因素，下列各项，能形成对照关系的一组是（　　）

完形填空

      Hey, I'm Simon. I have a strong and faithful dog, Bruce. The dog likes1with me. Sometimes he also follows my father to work. One morning when my father worked in the field, he2 his coat and put it under a big tree. He turned  to the dog and said,“3my coat, Bruce!” Bruce sat down on the coat. Later, my father remembered he had to buy some food.  4 he hurried to the shop.   5 shopping, he went home directly and forgot completely about his 6 and the dog. In the evening, I looked for my dog7 in the house, but I8 find him. At this time, my father suddenly remembered his coat and the dog. He went back to the tree   9. He found that Bruce was 10 sitting on the coat!