Explanation: Nitrogen (N₂) constitutes the major portion of the Earth's atmosphere by volume — about 78%. It is a colorless, odorless, and inert gas that plays a vital role in maintaining atmospheric stability and supporting life through the nitrogen cycle. Oxygen makes up about 21% of the atmosphere and is essential for respiration and combustion. Other gases like argon (~0.93%) and carbon dioxide (~0.04%) are present in much smaller amounts.

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.

Sir Isaac Newton invented the Reflecting Telescope in 1668. Unlike earlier refracting telescopes that used lenses, Newton’s design used a concave mirror to reflect and focus light. This eliminated chromatic aberration (color distortion) caused by lenses and significantly improved image clarity. Newton’s reflecting telescope became a major advancement in astronomical observation.

Other options (for context)

• Thermometer: Invented by Galileo Galilei (early version) and later improved by Daniel Gabriel Fahrenheit.
• Hydraulic Accumulator: Invented by William Armstrong.
• Transistor: Invented much later (in 1947) by John Bardeen, William Shockley, and Walter Brattain.

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: Athlete’s foot, scientifically known as Tinea pedis, is a fungal infection caused mainly by dermatophytes such as Trichophyton species. It thrives in warm, moist areas like the skin between the toes. Symptoms include itching, scaling, redness, and sometimes blistering. It spreads easily in communal areas like locker rooms and swimming pools. Proper hygiene and antifungal treatment can prevent and cure the infection.

Explanation: According to the Bureau of Indian Standards (BIS) – IS 10500:2012, the acceptable limit of total hardness in drinking water, expressed as CaCO₃, is 200 mg/l. The permissible limit in the absence of an alternate source is up to 600 mg/l. Excess hardness can cause scaling and affect the taste of water.

Explanation: RADAR (Radio Detection and Ranging) works by using radio waves to detect and determine the distance, speed, and position of objects such as aircraft, ships, and vehicles. The radar system transmits radio waves, which travel through the air, hit an object, and get reflected back to the radar receiver. By measuring the time delay between transmission and reception, the radar calculates the object's distance and motion. Radio waves are ideal because they can travel long distances, penetrate clouds, and are unaffected by weather conditions.