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: The acid present in the human stomach is hydrochloric acid (HCl). It helps in the digestion of food by breaking down proteins and activating the enzyme pepsin. HCl also kills harmful microorganisms present in the food, aiding in maintaining a healthy digestive system.

Explanation: A transformer is an electrical device used to increase (step-up) or decrease (step-down) the voltage of alternating current (A.C.). It works on the principle of electromagnetic induction and functions only with A.C., not with D.C. A transformer consists of two coils—primary and secondary—wound around a soft iron core. The voltage ratio between these coils depends on the number of turns in each winding. Transformers are used in power transmission, distribution, and electronic devices to regulate voltage levels efficiently.

Explanation: The transmission of light through optical fibres is based on the principle of Total Internal Reflection (TIR). When light travels from a denser medium (core of the fibre) to a rarer medium (cladding) and strikes the interface at an angle greater than the critical angle, it gets completely reflected back into the core instead of refracting out. This continuous internal reflection allows light to travel long distances through the fibre with minimal loss, even when the fibre is bent. Optical fibres are widely used in telecommunication, internet data transfer, and medical endoscopy.

Explanation: The flow of electrons through a metallic wire constitutes an electric current. In metals, free electrons move from the negative terminal to the positive terminal of a power source, while the conventional direction of current is taken as opposite — from positive to negative.

Explanation: A carnivore is an animal that primarily eats the flesh of other animals. The tiger (Panthera tigris) is a true carnivore and an apex predator in its ecosystem. It has sharp canines and retractable claws adapted for hunting and tearing flesh. Tigers play a vital role in maintaining the ecological balance by controlling herbivore populations in forests.

Explanation: Unwanted or unsolicited e-mails sent in bulk are called spam. These messages often contain advertisements, phishing links, or malware. They clutter inboxes and can pose security risks. The term “spam” originated from a comedy sketch by Monty Python, where the word was repeatedly used, much like unwanted messages.

Vitamin-B complex and Vitamin-C are water-soluble vitamins. This means they dissolve easily in water and are not stored in the body; hence, they must be consumed regularly through diet. Excess amounts of these vitamins are excreted through urine. Vitamin-B complex includes B₁ (Thiamine), B₂ (Riboflavin), B₃ (Niacin), B₆, B₁₂, and others that play key roles in metabolism and energy production. Vitamin-C (Ascorbic acid) helps in collagen formation, boosts immunity, and acts as an antioxidant.

Other options (for context)

• Vitamin-A & Vitamin-B: Vitamin-A is fat-soluble, while Vitamin-B is water-soluble.
• Vitamin-C & Vitamin-D: Vitamin-C is water-soluble, but Vitamin-D is fat-soluble.
• Vitamin-A & Vitamin-K: Both are fat-soluble vitamins.

Explanation: Insulin is a hormone secreted by the beta (β) cells of the Islets of Langerhans in the pancreas. Its main function is to regulate the blood glucose level by promoting the uptake of glucose into cells and converting excess glucose into glycogen in the liver and muscles. A deficiency or lack of insulin causes diabetes mellitus, leading to high blood sugar levels. The pancreas also secretes glucagon, which performs the opposite function by increasing blood glucose levels.