Considering the situation described above, after twenty years, they've found that the more one reduces pollution, the lower the marginal benefit.
This is because the marginal benefit of products, services, or conditions reduces as one continues to consume a specific product, service, or condition.
Therefore, as the country continues to reduce pollution, the marginal benefits decrease.
Hence, in this case, it is concluded that the correct answer is option A. "the lower the marginal benefit."
Learn more about marginal benefit here: https://brainly.com/question/18729570