Silver is the best conductor of heat among the given options. It efficiently transfers heat due to the presence of free electrons that move easily within its metallic structure. Because of this property, silver is used in high-quality thermal equipment and electrical contacts.

• Mercury: A metal but conducts heat less effectively than silver.
• Leather and Benzene: Poor conductors of heat (insulators).

The Moon takes about 27.3 days to complete one revolution around the Earth with respect to the stars (called the sidereal month). However, due to the simultaneous movement of the Earth around the Sun, the Moon takes approximately 29.5 days to complete one full cycle of phases (from new moon to new moon), known as the synodic month. This period is commonly rounded to 28 days in general usage.

Other options (for context)

• 45 days: Much longer than the Moon’s actual orbital period.
• 50 days: Incorrect — the Moon completes more than one revolution in this time.
• 65 days: Far too long compared to the real lunar cycle.

Explanation: The thyroid gland is the largest endocrine gland in the body. It is a butterfly-shaped gland situated in the neck region, just below the larynx. It secretes the hormones thyroxine (T₄) and triiodothyronine (T₃), which control metabolism, growth, and energy utilization. The thyroid also produces calcitonin, which helps regulate calcium levels in the blood.

Functions of the Thyroid Gland:

• Regulates metabolic rate (BMR)
• Influences growth and development
• Maintains body temperature and energy balance
• Regulates calcium metabolism via calcitonin

Disorders:

• Hypothyroidism: Leads to goitre, myxedema, or cretinism
• Hyperthyroidism: Leads to weight loss, anxiety, and increased heart rate

Explanation: CNG stands for Compressed Natural Gas. It is an eco-friendly fuel made primarily of methane (CH₄) and is used as an alternative to petrol and diesel. CNG burns cleanly, producing fewer pollutants and helping reduce air pollution.

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: Asbestos is a group of naturally occurring fibrous silicate minerals once widely used in construction and insulation. When inhaled, asbestos fibers accumulate in the lungs and pleura, leading to inflammation and scarring over years. This chronic exposure can cause asbestosis (lung fibrosis) and mesothelioma — a deadly cancer of the membrane lining the lungs (pleura) or abdomen (peritoneum).

Explanation: The main cause of global warming is the rise in the concentration of carbon dioxide (CO₂) and other greenhouse gases in the atmosphere. These gases trap heat radiated from the Earth’s surface, creating the greenhouse effect and leading to an overall increase in the planet’s temperature.

Explanation: Cow’s milk is a nutritionally rich food containing proteins, fats, carbohydrates, minerals, and vitamins. It is particularly rich in B-group vitamins, especially Vitamin B₂ (Riboflavin) and Vitamin B₁₂ (Cobalamin). These vitamins play crucial roles in maintaining healthy skin, eyesight, and the nervous system, as well as aiding in the formation of red blood cells.

Note: While milk provides moderate amounts of Vitamin A and D, the natural content of Vitamin D is low, and milk is often fortified with Vitamin D to enhance its nutritional value.

Explanation: During photosynthesis, green plants use sunlight to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂). The oxygen gas released in this process comes specifically from the splitting of water molecules — a reaction known as photolysis of water, which occurs in the light-dependent reaction inside the chloroplasts.

Chemical equation:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

The oxygen atoms in the released O₂ come entirely from the original water molecules, as demonstrated by isotopic experiments using heavy oxygen (O¹⁸) tracers.