Explanation: The heart is enclosed in a double-layered protective membrane called the pericardium. The outer fibrous layer protects the heart and anchors it within the chest cavity, while the inner serous layer secretes pericardial fluid that reduces friction during heartbeats. This fluid-filled sac ensures smooth and cushioned movement of the heart as it contracts and relaxes continuously.

Explanation: The rarest blood type in the world is AB⁻ (AB negative). It is found in less than 1% of the global population. People with AB⁻ blood can receive red blood cells from all Rh-negative blood types, but they can donate only to other AB individuals.

Explanation: The pairing Louis Pasteur : Blood groups is not correctly matched. Louis Pasteur is known for his work on pasteurization and germ theory of disease, not for discovering blood groups. The discovery of blood groups was made by Karl Landsteiner in 1900. Other matches are correct.

Explanation: The onion is a modified underground stem called a bulb. It consists of a short, disc-like stem (the basal plate) surrounded by concentric fleshy leaf bases that store food. The roots emerge from the underside of the basal plate, and new shoots grow from the center. Thus, the onion bulb serves as a storage organ for nutrients and helps in vegetative propagation.

Methane (CH₄) is not an allotrope of carbon. It is a chemical compound made up of one carbon atom and four hydrogen atoms. In contrast, allotropes are different structural forms of the same element in the same physical state. Carbon has several allotropes, including diamond, graphite, and fullerene (C-60), which differ in atomic arrangement and physical properties.

Other options (for context)

• Diamond: An allotrope of carbon where each carbon atom is tetrahedrally bonded to four others, making it extremely hard.
• Graphite: An allotrope of carbon with layers of hexagonally arranged atoms; good conductor of electricity.
• C-60 (Fullerene): A spherical allotrope of carbon consisting of 60 atoms arranged like a soccer ball.

The Kyoto Protocol is an international agreement related to the environment, specifically aimed at reducing greenhouse gas emissions that contribute to global warming and climate change. It was adopted in Kyoto, Japan, in 1997 and came into force in 2005. The protocol set binding targets for developed countries to limit or reduce their carbon dioxide (CO₂) and other greenhouse gas emissions.

Other options (for context)

• Nuclear Power: Not the focus of the Kyoto Protocol; this relates to energy generation and atomic energy regulation.
• Trade: Covered under agreements like GATT and WTO, not the Kyoto Protocol.
• None of these: Incorrect — the Kyoto Protocol deals specifically with environmental protection.

Explanation: Acids do not turn red litmus to blue; instead, they turn blue litmus paper red. Acids are sour in taste, corrosive in nature, and have a pH value less than 7. Therefore, the statement that acids turn red litmus to blue is incorrect.

The average distance between the Earth and the Sun is approximately 150 million kilometers (or about 1 Astronomical Unit – 1 AU). This distance serves as a standard unit of measurement for describing distances within our solar system. Despite slight variations due to Earth's elliptical orbit, this average remains nearly constant and is crucial for maintaining suitable conditions for life on Earth.

Other options (for context)

• 15 Million km: Far too close — the Earth would be extremely hot and uninhabitable.
• 90 Million km: Shorter than the actual average distance.
• 900 Million km: Much greater — about six times the real distance, closer to Saturn’s orbit.

Explanation: The phenomenon of radioactivity was discovered by the French physicist Henri Becquerel in 1896. He accidentally discovered that uranium salts emitted invisible rays that could fog photographic plates, even without sunlight. This led to the realization that certain elements spontaneously emit radiations — a property later termed as radioactivity. Subsequently, Marie and Pierre Curie expanded his work and discovered other radioactive elements like polonium and radium.

Explanation: The primary colours of light are Red, Green, and Blue (RGB). These are the fundamental colours that can be combined in different proportions to produce all other colours in the visible spectrum. Yellow is not a primary colour—it is a secondary colour formed by mixing red and green light. This principle of colour mixing is used in television, computer screens, and other digital displays that use RGB colour models.