Explanation: The major cause of global warming is the increasing concentration of carbon dioxide (CO₂) in the atmosphere. CO₂ is a greenhouse gas that traps heat radiating from the Earth's surface, preventing it from escaping into space — a phenomenon known as the greenhouse effect. Human activities such as the burning of fossil fuels, deforestation, and industrial emissions have significantly increased CO₂ levels, leading to a rise in the Earth’s average temperature, melting of glaciers, rising sea levels, and climate change.

Explanation: Chikungunya is a viral disease caused by the Chikungunya virus, which belongs to the Togaviridae family. It is transmitted to humans through the bite of infected Aedes aegypti and Aedes albopictus mosquitoes. The disease causes sudden fever, joint pain, headache, and rash. In contrast, cholera (caused by Vibrio cholerae), typhoid (by Salmonella typhi), and tuberculosis (by Mycobacterium tuberculosis) are all bacterial diseases.

Explanation: Kala-azar, also known as visceral leishmaniasis, is caused by the protozoan parasite Leishmania donovani. It is transmitted through the bite of the female Phlebotomus sandfly. The disease primarily affects the liver, spleen, and bone marrow, leading to symptoms such as fever, anemia, weight loss, and darkening of the skin. The term “Kala-azar” literally means “black fever.”

Explanation: Vitamin B₁₂, also known as Cobalamin, is the only vitamin that contains a metal ion — cobalt (Co) — at its core. It plays a crucial role in red blood cell formation, DNA synthesis, and proper nervous system function. Deficiency of Vitamin B₁₂ leads to pernicious anemia and neurological disorders. It is found mainly in animal-based foods such as meat, eggs, milk, and fish, and is synthesized by certain bacteria.

Explanation: Sodium hydroxide (NaOH) is a base, so it does not turn blue litmus paper red. Instead, it turns red litmus paper blue. In contrast, lemon juice, acetic acid, and hydrochloric acid are acidic and turn blue litmus red.

Explanation: Edward Jenner, an English physician, is known as the “Father of Immunology.” In 1796, he observed that milkmaids who had previously suffered from cowpox (a mild disease) did not contract smallpox (a deadly viral disease). He tested his hypothesis by injecting a boy with material taken from a cowpox sore and later exposing him to smallpox — the boy did not develop the disease. This led to the development of the first vaccine, derived from the Latin word vacca meaning “cow.”

Significance: Jenner’s discovery laid the foundation of modern immunology and preventive medicine. Smallpox was officially declared eradicated by the WHO in 1980.

Explanation: Below the critical temperature, gases can be liquefied by applying pressure because the intermolecular forces of attraction between gas molecules become effective. These forces pull the molecules closer together, resulting in the formation of a liquid state.

Explanation: Sound travels fastest in steel because it is a solid material where the particles are packed very closely together. Sound waves need a medium to travel, and their speed depends on how quickly vibrations can pass from one particle to another. In solids, particles are tightly bound, so vibrations are transmitted more efficiently. In contrast, sound travels slower in liquids and slowest in gases. It cannot travel through a vacuum because there are no particles to carry the vibrations.