Passage Summary: When sewage gets into calm coastal waters, it causes a chain reaction: extra nutrients lead to algae blooms, which feed tiny toxic creatures that kill off the fish.

Reasoning: Sewage pollution introduces nutrients into estuaries, which triggers algae growth that supports toxic microorganisms capable of killing fish.

Analysis: Since this is a Must be True question, we must treat the described causal chain as a series of absolute facts. The stimulus links sewage to overnutrification, overnutrification to algae, and algae to fish-killing toxins. Look for an answer that connects the beginning of this chain to the end, specifically how sewage can be the underlying cause of fish mortality. Avoid any choices that suggest this process happens every single time, as the stimulus uses the word 'sometimes' regarding the toxic microorganisms.