The Assertion (A) is true: In any food chain, the concentration of harmful chemicals like pesticides (e.g., DDT) is always highest at the top-most level. Tertiary consumers, such as hawks, tigers, or humans, sit at the end of the chain and thus harbor the most toxins.The Reason (R) is true: Pesticides are often non-biodegradable. When a primary consumer eats many plants, it absorbs all the pesticides from those plants. When a secondary consumer eats multiple primary consumers, it "collects" all the accumulated toxins from them. This process repeats up the chain.This process is called biomagnification. As we move up the food chain, the concentration of harmful chemicals increases because predators eat many organisms from the lower trophic level.