Explain why sometimes foaming is seen in river water near the place where sewage water is poured after treatment?

Options

1. ADetergents persist in water even after sewage treatment and cause foaming in river water.
2. BThe foam is a natural occurrence in clean river water.
3. CThe foam results from the increased oxygen levels in the treated sewage water.
4. DThe foam is due to the presence of dissolved gases in the river water.