There is some concern regarding the nanoparticles of some chemical elements that are used by the industry in the manufacture of various products. Why? 1. They can accumulate in the environment, and contaminate water and soil. 2. They can enter the food chains. 3. They can trigger the production of free radicals. Select the correct answer using the code given below.

Option (D) is correct because nanoparticles, due to their minute size and high reactivity, can persist in the environment, bioaccumulate across food chains, and trigger oxidative stress through free radical production. Option (A) is the most tempting wrong choice for candidates who may recognize the environmental persistence of these materials but overlook their specific biochemical toxicity at the cellular level. The core concept tested is the environmental and health risks (nanotoxicology) arising from the unique physical and chemical properties of nanomaterials.