Explanation: In a healthy adult, the total volume of blood in the body is approximately 7% of body weight. For an average person weighing around 70 kg, this equals about 5 to 5.5 litres of blood. Blood plays a vital role in transporting oxygen, nutrients, hormones, and waste products throughout the body. It also helps regulate temperature and maintain pH balance. The major components of blood are plasma (55%), red blood cells (RBCs), white blood cells (WBCs), and platelets.

Explanation: The processes that add carbon dioxide (CO₂) to the carbon cycle on Earth are:

• 1. Volcanic action: Volcanic eruptions release large amounts of CO₂ and other gases from the Earth’s interior into the atmosphere.
• 2. Respiration: All living organisms release CO₂ as a by-product of breaking down glucose during cellular respiration.
• 4. Decay of organic matter: Decomposition of dead plants and animals by microbes produces CO₂ and methane (CH₄).

Photosynthesis (option 3), on the other hand, removes CO₂ from the atmosphere as plants use it to produce glucose and release oxygen.

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.

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 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: Lysergic acid diethylamide (LSD) was discovered by Dr. Albert Hofmann, a Swiss chemist, in 1938 at the Sandoz Laboratories in Switzerland. LSD is a powerful hallucinogenic compound derived from lysergic acid, which occurs in the fungus Claviceps purpurea (ergot) that grows on rye. Hofmann accidentally discovered its psychological effects in 1943 when he absorbed a small amount through his skin, leading to vivid visual hallucinations. LSD acts on serotonin receptors in the brain and profoundly alters perception and mood.

Explanation: The tusks of elephants are actually modified second upper incisors. They are made primarily of dentine, commonly known as ivory, and covered by a thin layer of enamel at the tip (which wears off with time). Tusks grow continuously throughout the elephant’s life and are used for digging, lifting objects, stripping bark, defense, and dominance displays.

Note: Both male and female African elephants have tusks, whereas in Asian elephants, tusks are generally present only in males.

Composition: Tusks are composed of dentine (ivory) with fine concentric growth lines, and they grow from sockets in the skull.