补全对话。(5%) A: last Sunday? B: Yes, I did. My classmates and I went to plant trees. A: How did you plant trees? B: First, we in the ground. Then, we and put them into the holes. After that, we . We watered the young trees. A: ? B: Yes, they are. They can stop the wind and they can make the air fresh and clean. A: We should plant more trees.