Explanation: The elements Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) — collectively known as CHON — were primarily responsible for the origin of life on Earth. These elements are the basic building blocks of all biological molecules such as proteins, nucleic acids, carbohydrates, and lipids. Early Earth's atmosphere contained these elements in gaseous forms (like CH₄, NH₃, H₂, and H₂O), which combined under lightning and UV radiation to form simple organic compounds — a process explained by the Miller–Urey experiment.

Explanation: Inside a sealed pressure cooker, the pressure is higher than atmospheric pressure, which raises the boiling point of water to about 121℃. This allows food to cook faster because water and steam reach higher temperatures than at normal atmospheric pressure.

Explanation: Measles is a highly contagious viral disease caused by the measles virus (a member of the Paramyxovirus group). It spreads through droplets released when an infected person coughs or sneezes. Symptoms include fever, cough, rash, and watery eyes. Other diseases listed — scurvy, asthma, and diabetes — are non-communicable as they do not spread from person to person.

Explanation: Chlorofluorocarbons (CFCs) are the gases responsible for the depletion of the ozone layer. When released into the atmosphere, CFCs break down under ultraviolet (UV) radiation, releasing chlorine atoms that destroy ozone molecules (O₃), leading to thinning of the ozone layer and increased UV radiation reaching the Earth.

The full form of LAN is Local Area Network. It is a network that connects computers and devices within a limited geographical area such as a home, office, school, or building. LANs enable data sharing, resource sharing (like printers or internet connections), and communication between connected devices. Common technologies used in LANs include Ethernet and Wi-Fi.

Other options (for context)

• Line Area Network: Incorrect — not a recognized networking term.
• Linear Area Network: Incorrect — there is no such standard network type.
• Land Area Network: Incorrect — misinterpretation of LAN abbreviation.

Explanation: A solar eclipse occurs when the Moon comes between the Sun and the Earth and casts its shadow on the Earth. This happens during the new moon phase, when all three celestial bodies are almost in a straight line. Depending on the alignment, the eclipse can be total (Sun completely covered), partial (Sun partially covered), or annular (ring-shaped Sun visible). During this event, sunlight is blocked for a short time in specific areas of the Earth’s surface.

Sustainable Development refers to growth that meets the needs of the present without compromising the ability of future generations to meet their own needs. It aims to balance economic progress, social equity, and environmental protection by conserving natural resources and reducing pollution for long-term ecological stability.

• Green economy: Focuses mainly on eco-friendly industries.
• Holistic development: Emphasizes overall human and social growth.
• Subsistence agriculture: Involves farming mainly for self-consumption.

Explanation: The sour taste of a lemon is due to the presence of citric acid. Citric acid is a weak organic acid found naturally in citrus fruits such as lemons, oranges, and limes. It gives them their characteristic tangy flavor and also acts as a natural preservative.

Explanation: Saliva, secreted by the salivary glands, begins the process of carbohydrate digestion in the mouth. It contains the enzyme salivary amylase (also known as ptyalin), which catalyzes the breakdown of starch (polysaccharide) into maltose (disaccharide). This is the first step in digestion, and it occurs while food is chewed and mixed with saliva before being swallowed.

Chemical Reaction:

Starch → (Salivary amylase) → Maltose

Thus, saliva plays an essential role in the digestion of carbohydrates, making the food slightly sweet when chewed for a longer time.

Explanation: Uranium-235 (U-235) is the isotope used for the production of atomic energy because it is fissionable. When bombarded with neutrons, U-235 undergoes nuclear fission, releasing a large amount of energy along with additional neutrons, which sustain the chain reaction in nuclear reactors and atomic bombs.