Eutrophication sequence in nutrient-polluted lakes

Steps:

• Nitrogen fertilizer enriches water, promoting rapid growth of green algae and plants that cover the surface (3).
• Excessive growth leads to plant death due to lack of light and nutrients lower down (4).
• Dead plants are broken down by decomposers, which consume oxygen in the process (2).
• Oxygen depletion causes hypoxic conditions in the lake (1).

Why C is correct:

• Eutrophication defines nutrient pollution causing algal blooms (3), die-off (4), microbial decomposition (2), and oxygen depletion (1).

Why the others are wrong:

• A: Oxygen decrease (1) cannot precede plant growth (3), as it results from later decomposition.
• B: Decomposers (2) require dead plants (4) first, not initial oxygen drop.
• D: Plant death (4) follows growth (3), not vice versa.

Final answer: C