The correct answer is: D. Bacteria and colloidal solids.
Rapid gravity filters are a type of water treatment system that uses a bed of sand or gravel to remove suspended particles and bacteria from water. The water flows through the filter bed at a high rate, which causes the particles to be trapped in the pores of the sand or gravel. The bacteria are also removed by the filter, as they are too large to pass through the pores.
Dissolved organic substances are not removed by rapid gravity filters, as they are too small to be trapped in the pores of the sand or gravel. Dissolved gases are also not removed by rapid gravity filters, as they pass through the filter bed without being affected. Floating solids are removed by rapid gravity filters, as they are too large to pass through the pores of the sand or gravel. Dissolved inorganic solids are not removed by rapid gravity filters, as they are too small to be trapped in the pores of the sand or gravel.
Rapid gravity filters are a simple and effective way to remove bacteria and colloidal solids from water. They are commonly used in municipal water treatment plants and in homes with private wells.