The woods behind the high school have become littered with trash. The students at the school are partly responsible for the problem. We are often careless with our trash when we are in the schoolyard. Since we helped make the mess, we should organize a clean-up day. Which sentence from the paragraph best identifies the letter’s purpose?

The woods behind the high school have become littered with trash.
As students at the school, we are partly responsible for the problem.
We are often careless with our trash when we are in the schoolyard.
Since we helped make the mess, we should organize a clean-up day.