根据对话内容，从下面方框中选择适当的单词填空，使对话内容完整正确。

Reporter：Can you introduce something about your latest class activity?
Monitor：Sure. The students from our class planted some lines of trees on March 12^th, 2008.
Reporter：How did you plant trees?
Monitor：First, the students were                                into groups of four. Next, each group of students planted a tree. Then, they poured some water over the trees.
Reported：How do you         for the trees?
Monitor： We plan to come back and look after the trees                        .
Reported：Sounds good. Could you tell me why you choose to do that?
Monitor：As you know, pollution has become one of the most serious       . We would like to try our best to improve our environment. 
Reporter：Well, it’s very meaningful. If everyone can do something to protect the environment        , our city will become much more beautiful.