Explanation: The hard structure of bones and teeth is primarily made of calcium phosphate in the crystalline form of tricalcium phosphate (Ca₃(PO₄)₂). It provides rigidity and strength. In tooth enamel, a small portion is converted into fluorapatite, which makes teeth resistant to bacterial decay.

Explanation: Ascorbic acid is not a member of the vitamin B complex; it is actually vitamin C. The vitamin B complex includes vitamins like thiamine (B₁), riboflavin (B₂), niacin (B₃), folic acid (B₉), and others that play important roles in metabolism and energy production.

Explanation: When ice floats in water, it displaces an amount of water equal to the weight of the ice. As the ice melts, it converts into the same amount of water, exactly filling the displaced volume. Therefore, the water level remains unchanged after all the ice melts.

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.

Silviculture is the branch of forestry concerned with the management, cultivation, and care of forests. It involves the practice of controlling the establishment, growth, composition, and quality of forest stands to meet diverse needs such as timber production, wildlife habitat, and environmental conservation. The term comes from Latin — silva meaning forest, and cultura meaning cultivation.

Other options (for context)

• Rivers: Managed under hydrology or water resource management, not silviculture.
• Lakes: Managed under limnology or aquatic resource management.
• Hills: May be part of forest areas, but silviculture specifically refers to forest management.

Xylem is a type of vascular tissue in plants responsible for the transportation of water and dissolved minerals from the roots to the stems and leaves. The upward movement of water through xylem vessels occurs due to processes like transpiration pull, root pressure, and capillary action. Xylem also provides mechanical support to the plant.

Other options (for context)

• Food: Transported by phloem, not xylem.
• Nutrient: Minerals are carried along with water but the primary function of xylem is water transport.
• Both food and water: Incorrect — only phloem transports food; xylem transports water.

A Lunar Eclipse occurs when the Earth comes between the Sun and the Moon, causing the Earth’s shadow to fall on the Moon. This alignment blocks the sunlight that normally reflects off the Moon’s surface, making it appear dark or reddish (known as a Blood Moon). Lunar eclipses only occur during a full moon when the Sun, Earth, and Moon are in a straight line (or nearly so).

Other options (for context)

• When Sun is between Earth and Moon: Incorrect — this alignment causes no eclipse.
• When Moon is between Earth and Sun: This alignment causes a solar eclipse, not a lunar one.
• When Earth is between Sun and other celestial bodies: Too general — lunar eclipse specifically involves the Moon.

Explanation: Plants absorb nitrogen from the soil mainly in the form of nitrates (NO₃⁻). Inside the plant, nitrates are reduced to nitrites (NO₂⁻) and then to ammonia (NH₃) through the action of specific reductase enzymes. The ammonia thus formed is then used to synthesize amino acids, proteins, nucleic acids, and other nitrogen-containing compounds necessary for plant growth.

Simplified Pathway:

NO₃⁻ → NO₂⁻ → NH₃ → Amino acids → Proteins