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.