Pesticides in water sources are alarming because they biomagnify in food chains and may have unknown effects on organisms.

• A. They are quickly broken down by sunlight and pose no risk.
• B. They biomagnify in food chains and may have unknown effects on organisms.
• C. They only affect pests and do not harm beneficial species.
• D. They do not persist in water after rainfall.

Answer: (B)

Pesticides entering water bodies can persist in the environment and accumulate in organisms, so their concentrations can rise as they move up the food chain. This leads to biomagnification, where predators end up with much higher internal doses than their prey, increasing the risk of harm to wildlife and potentially humans who consume those higher-trophic-level organisms. The idea that these chemicals may have unknown effects on organisms reflects how chronic exposure, sublethal impacts (like endocrine disruption or altered behavior), and the combined effects of multiple pesticides can be difficult to predict from short-term tests.

That’s why the statement about biomagnification and potential unknown effects best captures the real ecological risk. The other ideas—rapid breakdown by sunlight implying no risk, only affecting pests and sparing beneficial species, or not persisting in water after rainfall—do not align with how many pesticides behave in the environment, including persistence, non-target effects, and movement through waterways and ecosystems.