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.

Glaciers usually melt first at the bottom. The pressure at the base of a glacier is very high, which lowers the melting point of ice — a phenomenon known as pressure melting. Additionally, heat from the Earth’s crust and friction caused by glacier movement also contribute to faster melting at the base.

• Top surface: Exposed to air but melts slower due to lower pressure.
• Sides and middle surface: Experience less pressure and friction, so melting is slower.

Explanation: When a man inside a moving train tosses a coin and it falls behind him, it means the train is moving forward with acceleration. The coin, after being tossed, continues to move with the train’s initial velocity, but since the train speeds up, the coin appears to fall backward relative to the man.

Explanation: Stars appear to twinkle at night due to the refraction of light by the Earth's atmosphere. As starlight passes through different layers of air with varying densities, its path bends continuously, causing the light’s intensity and position to appear to change, creating the twinkling effect.

The normal body temperature of a healthy human being is approximately 37 °C, which is equivalent to 98.6 °F. This temperature is maintained through a balance between heat production and heat loss in the body. The body’s thermoregulation is controlled by the hypothalamus in the brain to ensure proper functioning of enzymes and metabolic processes.

Other options (for context)

• 98.4 °C: Incorrect — the correct equivalent in Fahrenheit is 98.6 °F, not in Celsius.
• 14.5 °C: Much lower than the human body temperature — such a value would indicate hypothermia or death.
• 25 °C: Represents a mild room temperature, not body temperature.

Running water is the source of hydel power (also called hydroelectric power). The energy of flowing or falling water is converted into electrical energy using turbines and generators in dams. It is a renewable, clean, and sustainable source of energy widely used in India and worldwide.

• Thermal power: Generated by burning coal or other fuels.
• Nuclear power: Produced from nuclear fission reactions.
• None of the above: Incorrect — running water directly produces hydel power.

A rocket functions based on Newton’s Third Law of Motion, which states that “for every action, there is an equal and opposite reaction.” The rocket expels gases downward at high speed (action), and in response, the rocket is propelled upward (reaction). This principle enables space rockets to move even in the vacuum of space where no air resistance exists.

• Newton’s First Law: Deals with inertia, not propulsion.
• Newton’s Second Law: Explains force and acceleration but doesn’t directly describe rocket thrust.