The term 'oxygen sag curve' describes what?

• A. A measure of bacterial growth rate
• B. A graph showing a rise in DO downstream after pollution
• C. An index of water salinity
• D. The oxygen sag curve describes a graph showing DO changes with a drop after organic waste enters water and recovery downstream

Answer: (D)

The main idea is how dissolved oxygen changes along a river when organic pollution is introduced. When organic waste enters water, bacteria break it down, which uses a lot of oxygen and creates a high biological oxygen demand. This causes a dip in dissolved oxygen downstream of the pollution source—the sag. As the water flows farther downstream, the pollution is diluted and further decomposed, and oxygen from the air re-enters the water, so the DO levels recover toward normal. That pattern—an initial drop in DO followed by gradual recovery downstream—is exactly what the term describes.

The other options don’t fit: one is about bacterial growth rate, not DO patterns; another suggests DO rises after pollution (unusual in typical oxygen sag scenarios); and one is about salinity, which isn’t related to DO changes from organic waste.