Explanation: The filament of an electric bulb is made of tungsten because it has a very high melting point of about 3380°C and good tensile strength. When electric current passes through it, tungsten glows brightly without melting, converting electrical energy into light and heat. It also has low vapour pressure, which prevents quick evaporation of the filament even at high temperatures, ensuring longer bulb life.

Explanation: The gas used for artificial ripening of green fruits is ethylene (C₂H₄). Ethylene is a natural plant hormone that regulates fruit ripening by stimulating the conversion of starches into sugars, making the fruit softer and sweeter. In artificial ripening, ethylene gas is introduced in controlled amounts to uniformly ripen fruits such as bananas, mangoes, and tomatoes. However, the use of harmful chemicals like calcium carbide for ripening is banned because it produces toxic substances like acetylene.

Explanation: The main function of the kidneys is to purify blood by removing waste products, excess salts, and extra water from the body in the form of urine. Each kidney contains about one million nephrons, which are the structural and functional units responsible for blood filtration. The kidneys also help in maintaining the acid-base balance, water balance, and blood pressure, and play a role in hormone secretion such as erythropoietin (for RBC production).

Sodium bicarbonate (NaHCO₃), also known as baking soda, is used in fire extinguishers. When heated, it decomposes to produce carbon dioxide (CO₂), which displaces oxygen and helps smother the fire. It is especially effective for extinguishing electrical and small grease fires.

• Sodium carbonate: Used in glass and soap making, not for fire control.
• Sodium nitrate: An oxidizing agent, not suitable for extinguishing fires.
• Sodium sulphate: Used in detergents and paper making, not in fire extinguishers.

Nitrogen dioxide (NO₂) is a major cause of acid rain. When released into the atmosphere from vehicle exhausts and industrial emissions, it reacts with water vapor to form nitric acid (HNO₃). Along with sulfur dioxide forming sulfuric acid, it contributes to acid rain, which damages vegetation, aquatic life, and buildings.

• Carbon dioxide: Causes global warming, not acid rain.
• Carbon monoxide: A toxic gas but not acidic.
• Oxygen: Essential for respiration, not a pollutant.

Explanation: Mucor is not a biofertilizer; it is a saprophytic fungus that grows on decaying organic matter. Biofertilizers are living microorganisms that enhance soil fertility by fixing atmospheric nitrogen or solubilizing nutrients. Examples include Rhizobium, Azotobacter, and Anabaena.

Explanation: The main constituents of biogas are methane (CH₄) and carbon dioxide (CO₂). Biogas is produced through the anaerobic decomposition of organic matter such as animal dung and plant waste. Methane acts as the primary combustible component, making biogas an efficient and renewable source of energy.

Explanation: Gerontology is the branch of medical and biological science that deals with the study of aging and the diseases and problems specific to old age. It focuses on understanding the physical, mental, and social changes that occur in elderly individuals, as well as methods to promote healthy aging. Gerontology includes aspects of geriatrics, which specifically involves the medical care and treatment of elderly patients.

Explanation: Bullets are typically made of lead or lead alloys because lead is dense and malleable. When bullets remain lodged in the body, they can corrode over time and release lead ions into surrounding tissues and blood, resulting in chronic lead poisoning (plumbism). Symptoms include abdominal pain, anemia, fatigue, headache, irritability, and neurological disorders. In severe cases, it may cause kidney and brain damage.

Prevention: Complete removal of bullet fragments and regular monitoring of blood lead levels in affected individuals are essential to prevent toxicity.