Insulin was discovered by Frederick Banting and Charles Best in 1921 at the University of Toronto. Banting and his team successfully isolated insulin from the pancreas, which became a breakthrough in the treatment of diabetes mellitus. For this discovery, Frederick Banting and J.J.R. Macleod were awarded the Nobel Prize in Physiology or Medicine in 1923.

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• Jonas E. Salk: Discovered the first successful polio vaccine.
• Alexander Fleming: Discovered penicillin, the first antibiotic.
• Roentgen: Discovered X-rays in 1895.

Explanation: The element hydrogen is common to all acids. Acids are substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. This property gives acids their characteristic sour taste, the ability to turn blue litmus paper red, and to react with bases to form salts and water. Examples include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃)—all of which release hydrogen ions in aqueous solution.

Explanation: Sodium (Na⁺) plays a crucial role in the transmission of nerve impulses in the human body. Nerve cells (neurons) transmit signals through an electrochemical process known as the nerve impulse or action potential. During this process, the movement of sodium (Na⁺) and potassium (K⁺) ions across the nerve cell membrane generates an electrical charge. The rapid influx of sodium ions causes depolarization, which allows the nerve impulse to travel along the neuron, ensuring proper communication between the brain, spinal cord, and body.

Explanation: The term LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser produces a highly coherent, monochromatic, and intense beam of light by amplifying light through the process of stimulated emission. In this process, atoms or molecules in an excited state emit photons in phase with incoming photons, resulting in a strong, directed beam. Lasers are widely used in medicine, communication, industry, defense, and scientific research.

The Taj Mahal is threatened primarily due to sulphur dioxide (SO₂) pollution. When sulphur dioxide released from nearby industries and vehicles mixes with moisture in the air, it forms sulphuric acid, leading to acid rain. This acid rain reacts with the white marble of the Taj Mahal, causing it to turn yellow and deteriorate — a phenomenon known as “Marble Cancer.”

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• Carbon dioxide: Contributes to global warming but does not directly damage marble.
• Carbon monoxide: Harmful to health but has minimal effect on marble structures.
• Water vapour: Naturally present in the atmosphere and not harmful to monuments.