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

Steps:

• Identify normal dissolved oxygen levels in rivers, typically stable or gradually decreasing downstream.
• Look for a sudden decrease in oxygen concentration, indicating organic pollution like sewage.
• Note the point of sharpest drop, as bacteria consume oxygen to break down sewage.
• Confirm recovery downstream if reaeration occurs.

Why B is correct:

• Point B shows the abrupt decline in dissolved oxygen, matching the biological oxygen demand (BOD) increase from sewage decomposition.

Why the others are wrong:

• A: Upstream point with stable or high oxygen, no pollution input.
• C: Post-dip recovery phase, oxygen rising due to natural reaeration.
• D: Downstream stabilization, no new sewage addition.

Final answer: B