Explanation: Albert Einstein received the Nobel Prize in Physics in 1921 for his discovery of the photoelectric effect, not for his famous theory of relativity. In the photoelectric effect, when light of a certain frequency falls on a metal surface, it emits electrons. Einstein explained this phenomenon by proposing that light is made up of discrete packets of energy called photons. The energy of each photon is given by E = hν, where h is Planck’s constant and ν is the frequency of light. This discovery laid the foundation for quantum physics and modern electronics.

Explanation: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

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

Explanation: The blue whale (Balaenoptera musculus) holds the record as the largest mammal and the largest living organism on Earth. Adult blue whales can reach lengths of up to 30 meters (98 feet) and weigh over 180 tons. Despite their size, they feed mainly on small shrimp-like creatures called krill, filtering them through their baleen plates. Whales are warm-blooded, breathe air through blowholes, and give birth to live young — all key mammalian traits.

Comparison:

• Blue whale (largest mammal): up to 30 m long, ~180 tons
• African elephant (largest land mammal): up to 4 m tall, ~6 tons
• Rhinoceros: up to 2 m tall, ~2 tons

Sericulture is the rearing and cultivation of silkworms for the production of silk. The process involves raising silkworms on mulberry leaves, harvesting their cocoons, and then extracting silk fibers from these cocoons. The most commonly used silkworm species is Bombyx mori. Sericulture is an important cottage industry in countries like India, China, and Japan.

Other options (for context)

• Fishes: Their rearing is called Pisciculture.
• Honey bees: Their rearing is called Apiculture.
• Earth worms: Their cultivation for composting is called Vermiculture.

Carbon dioxide (CO₂) is commonly used as a fire extinguisher gas. It works by displacing oxygen around the fire and forming a blanket of gas that cuts off the oxygen supply, thereby stopping the combustion process. Additionally, CO₂ is non-flammable and leaves no residue, making it ideal for use on electrical and electronic fires where water cannot be used safely.

Other options (for context)

• Nitrogen dioxide (NO₂): A toxic gas, not suitable for firefighting.
• Carbon monoxide (CO): A poisonous gas produced by incomplete combustion, not used in extinguishers.
• Nitrous oxide (N₂O): Commonly known as laughing gas, used as an anesthetic, not a fire suppressant.

Explanation: Nucleons are the particles that make up the nucleus of an atom — namely, protons and neutrons. A positron is not a nucleon; it is the antiparticle of the electron, having the same mass as an electron but a positive charge, and exists outside the nucleus.