The correct answer is D. a-1, b-4, c-3, d-2.
DDT is a pesticide, NaClO3 is a disinfectant, Cl2 is a bleaching agent, and PAN is a component of photochemical smog.
Pesticides are chemicals used to kill pests, such as insects, rodents, and weeds. They can be classified into several types, including insecticides, herbicides, and fungicides. DDT is a type of insecticide that was once widely used to control mosquitoes and other pests. However, it has since been banned in many countries due to its harmful effects on the environment and human health.
Disinfectants are chemicals used to kill or inactivate microorganisms, such as bacteria and viruses. They are often used to clean surfaces and equipment in hospitals and other healthcare settings. NaClO3, also known as sodium hypochlorite, is a common type of disinfectant.
Bleaching agents are chemicals used to whiten or bleach fabrics, paper, and other materials. They often contain chlorine, which is a powerful oxidizing agent. Cl2 is a type of bleaching agent.
Photochemical smog is a type of air pollution that forms when sunlight reacts with pollutants in the atmosphere. It is a major problem in urban areas, where it can cause respiratory problems, heart disease, and cancer. PAN is a component of photochemical smog.