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: Vitamin B₂ is also known as Riboflavin. It is a water-soluble vitamin that plays a vital role in the body’s energy production and cellular function. Riboflavin acts as a coenzyme in the metabolism of carbohydrates, fats, and proteins, helping to convert food into energy. It is also essential for maintaining healthy skin, eyes, and nerve functions. A deficiency of Vitamin B₂ can lead to cracked lips, sore throat, and inflammation of the tongue (a condition known as ariboflavinosis).

Explanation: The thyroid gland is located in the neck region, just below the larynx (voice box) and in front of the trachea (windpipe). It is a butterfly-shaped endocrine gland that secretes the hormones thyroxine (T₄) and triiodothyronine (T₃). These hormones regulate metabolism, growth, and energy production in the body. The thyroid also produces calcitonin, which helps control calcium levels in the blood. Iodine is essential for the synthesis of thyroid hormones.

Aluminium (Al) is the most abundant metal in the Earth's crust, making up about 8.2% by weight. It occurs mainly in the form of bauxite ore (Al₂O₃·2H₂O). Despite being highly abundant, aluminium is never found in its pure form naturally due to its strong reactivity with oxygen. It is widely used in industries for making utensils, aircraft parts, electrical wires, and packaging materials.

Other options (for context)

• Zinc: Found in smaller quantities; used in galvanization and alloys.
• Copper: Less abundant; mainly used in electrical wiring.
• Iron: Very abundant but ranks below aluminium in crustal abundance.

Explanation: Humidity is a measure of the amount of water vapour (moisture) present in the air. It indicates how saturated the air is with moisture and affects weather, comfort, and precipitation. High humidity means the air contains more water vapour, while low humidity indicates dry air.

One molecule of a Chlorofluorocarbon (CFC) can destroy up to 1,00,000 molecules of ozone (O₃) in the ozone layer. When CFCs reach the stratosphere, they are broken down by ultraviolet (UV) radiation, releasing chlorine atoms. Each chlorine atom acts as a catalyst in the breakdown of ozone molecules, significantly depleting the protective ozone layer that shields Earth from harmful UV rays.

Other options (for context)

• 10,000: Underestimates the destructive potential of a single CFC molecule.
• 10,00,000: Overestimates the number of ozone molecules destroyed.
• No one of the above: Incorrect — the correct answer is 1,00,000 molecules of ozone.

Explanation: The pyloric sphincter is a circular band of smooth muscle located at the junction of the stomach and the duodenum (first part of the small intestine). It regulates the passage of partially digested food (chyme) from the stomach into the small intestine and prevents backflow. The cardiac sphincter, by contrast, controls food entry into the stomach from the esophagus.

Explanation: Heavy water is chemically known as Deuterium oxide (D₂O). It is similar to ordinary water (H₂O), but the hydrogen atoms are replaced by deuterium — a heavier isotope of hydrogen containing one proton and one neutron. Heavy water is not suitable for drinking but is used as a moderator and coolant in nuclear reactors because it slows down neutrons, allowing a controlled nuclear fission reaction. It is also used in scientific research and isotope studies.