Option 4: Since we helped make the mess, we should organize a clean-up day.
The letter's purpose is to show that the woods' pollution was caused by the students and call them to action: to gather and organize a clean-up day.
- First, the letter presents the problem: The woods nearby the school are polluted.
- Second, it shows the cause of it: The students are partly responsible, they are careless.
- Third, it states a logical conclusion: They made the mess, they should organize to clean it all up.