Sewage input causes a sharp drop in dissolved oxygen due to bacterial decomposition.

Steps:

• Identify normal dissolved oxygen levels in rivers, typically stable or slightly decreasing downstream.
• Look for a sudden decrease in oxygen concentration, indicating organic pollution.
• Recognize that sewage introduces biodegradable matter, increasing biochemical oxygen demand (BOD).
• Pinpoint the location of the sharp dip as the sewage entry point.

Why B is correct:

• Option B shows the abrupt drop in dissolved oxygen, matching the biological oxygen demand (BOD) effect where bacteria consume oxygen to break down sewage organics.

Why the others are wrong:

• A: Oxygen levels are stable or high, indicating clean upstream water without pollution.
• C: Oxygen is recovering post-dip, showing natural reaeration downstream of pollution.
• D: Levels stabilize or rise further, reflecting full recovery far from the input.

Final answer: B