Explanation: The Sun is seen a few minutes before actual sunrise and after actual sunset due to the refraction of light by the Earth's atmosphere. The atmosphere bends the Sun’s rays towards the Earth, making the Sun appear slightly above the horizon even when it is below it.

Explanation: Minamata disease was caused by severe mercury poisoning in the coastal city of Minamata, Japan. It resulted from the discharge of methyl mercury compounds into Minamata Bay by a chemical factory (Chisso Corporation) during the 1950s and 1960s. The toxic mercury accumulated in fish and shellfish, which were later consumed by local residents. Symptoms included numbness, muscle weakness, loss of vision, hearing and speech, paralysis, and in severe cases, death. This incident is one of the world’s worst cases of industrial pollution.

Explanation: White Blood Cells (WBCs), also known as leukocytes, act as the body’s defense system. They protect the body against pathogens such as bacteria, viruses, and fungi. Different types of WBCs — like lymphocytes, monocytes, neutrophils, eosinophils, and basophils — have specialized roles in immunity. Because they “fight” infections, they are popularly called the ‘soldiers of the human body.’

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.

The Ozone layer in the Earth’s stratosphere protects living organisms from the Sun’s harmful ultraviolet (UV) rays. Ozone (O₃) absorbs most of the high-energy UV radiation, preventing it from reaching the Earth’s surface, where it could cause skin cancer, cataracts, and damage to plants and marine ecosystems. This protective layer is mainly found between 10 to 50 km above the Earth’s surface.

Other options (for context)

• Oxygen: Essential for respiration but does not block UV rays.
• Carbon dioxide: A greenhouse gas responsible for trapping heat, not UV protection.
• Nitrogen: Makes up most of the atmosphere but plays no role in UV absorption.

Explanation: The troposphere is the lowest layer of the Earth's atmosphere and is closest to the Earth's surface. It extends up to about 8–15 km from the surface, depending on latitude and season. This layer contains about 75% of the total atmospheric mass and almost all the water vapor and weather phenomena such as clouds, rain, and storms occur here. The temperature decreases with increasing altitude in this layer. The boundary between the troposphere and the stratosphere is known as the tropopause.

Explanation: Brass, an alloy of copper and zinc, gets discoloured when exposed to air containing hydrogen sulphide (H₂S). The H₂S reacts with the metal surface to form a dark layer of metal sulphides (mainly copper sulphide), causing tarnishing and loss of shine.

Oxygen is transported to the body’s cells by Red Blood Cells (RBCs). RBCs contain a special iron-containing pigment called hemoglobin, which has the ability to bind oxygen in the lungs and release it in body tissues where it is needed for cellular respiration. This process ensures that all body cells receive the oxygen required to produce energy (ATP).

Other options (for context)

• White Blood Cells (WBCs): Defend the body against infections and diseases, but do not carry oxygen.
• Hormones: Chemical messengers that regulate body functions, not involved in oxygen transport.
• Enzymes: Biological catalysts that speed up chemical reactions, not oxygen carriers.