Explanation: Water shows an unusual property known as anomalous expansion. Its density is maximum at 4°C. When cooled from higher temperatures, water contracts till 4°C, becoming denser. Below 4°C, it expands instead of contracting, so its density decreases. This property allows ice to float and prevents water bodies from freezing solid, protecting aquatic life during winters.

Sound becomes hazardous and causes noise pollution when its intensity exceeds 80 decibels (dB). Prolonged exposure to such high noise levels can lead to hearing loss, stress, sleep disturbances, and other health problems. Common sources include heavy traffic, loudspeakers, machinery, and construction activities. Sounds above 120 dB can cause immediate ear damage or pain.

Other options (for context)

• Above 50 dB: Considered moderate — typical of normal conversation or office noise.
• Above 100 dB: Very loud and potentially painful, but 80 dB is the threshold for long-term hazard.
• Above 130 dB: Extremely loud, causes immediate physical pain and ear damage.

All of the above are examples of static electricity. Static electricity is the result of the accumulation of electric charges on the surface of objects. When materials like glass, silk, ebonite, or wool are rubbed together, electrons are transferred, creating a charge imbalance. Similarly, lightning is a large-scale natural discharge of static electricity between clouds and the Earth.

• Glass rod & silk cloth: Positive and negative charges develop.
• Ebonite rod & wool: Negative charge accumulates on ebonite.
• Lightning: Discharge of atmospheric static electricity.

Explanation: Boron is a vital micronutrient that assists in the uptake and utilization of calcium and maintains the integrity of cell walls and membranes. It also regulates water movement in plant tissues and plays a role in pollen germination and fruit development. Deficiency of boron can lead to poor calcium translocation, brittle tissues, and reduced water absorption.

Neil Armstrong was the first man to land on the Moon on July 20, 1969, during NASA’s Apollo 11 mission. He became the first human to step onto the lunar surface, famously saying, "That's one small step for man, one giant leap for mankind." His historic achievement marked a major milestone in space exploration.

Other options (for context)

• Aldrin: Refers to Buzz Aldrin, the second man to walk on the Moon, who accompanied Armstrong on Apollo 11.
• Michael Collins: The third member of the Apollo 11 crew, who orbited the Moon in the command module but did not land.
• Gagarin: Refers to Yuri Gagarin, the first human to travel into space in 1961, but he never landed on the Moon.

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: ELISA stands for Enzyme-linked immunosorbent assay. It is a biochemical test used to detect and measure antibodies, antigens, proteins, or hormones in a sample. The method relies on an enzyme-linked antigen or antibody and a color change to indicate a positive reaction.

Explanation: All enzymes are proteins (except a few RNA-based enzymes called ribozymes). Enzymes act as biological catalysts that speed up chemical reactions in living organisms without being consumed in the process. They work by lowering the activation energy of reactions, ensuring that metabolic processes occur efficiently at normal body temperatures. Each enzyme is specific to a particular substrate, forming an enzyme-substrate complex to carry out its function effectively.

Metals can be beaten into thin sheets because they are malleable. Malleability is a physical property of metals that allows them to withstand hammering or rolling without breaking. This property is due to the metallic bonding where positive metal ions are surrounded by a ‘sea’ of delocalized electrons, allowing the layers of atoms to slide over each other without breaking the bond. Examples of highly malleable metals include gold, silver, and aluminium.

Other options (for context)

• Ductile: Refers to the ability of metals to be drawn into wires, not beaten into sheets.
• Soft: Not true for all metals — many are hard yet malleable.
• Loosely arranged particles: Incorrect — metal atoms are closely packed in a lattice structure.