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.

A fuse wire has high resistance and a low melting point. It is made of materials like tin-lead alloy so that it melts quickly when excessive current flows through the circuit. This breaks the circuit and prevents damage to electrical appliances due to overheating or short circuits.

• High resistance: Causes heating when excess current passes.
• Low melting point: Ensures the fuse melts easily to interrupt the circuit.

Explanation: The longest day in the Southern Hemisphere occurs on 22nd December, which marks the Summer Solstice in that hemisphere. On this date, the South Pole is tilted toward the Sun, resulting in longer daylight hours and shorter nights. Conversely, at the same time, the Northern Hemisphere experiences its winter solstice—the shortest day and longest night of the year. After 22nd December, the days in the Southern Hemisphere gradually begin to shorten.

Explanation: Bullets are typically made of lead or lead alloys because lead is dense and malleable. When bullets remain lodged in the body, they can corrode over time and release lead ions into surrounding tissues and blood, resulting in chronic lead poisoning (plumbism). Symptoms include abdominal pain, anemia, fatigue, headache, irritability, and neurological disorders. In severe cases, it may cause kidney and brain damage.

Prevention: Complete removal of bullet fragments and regular monitoring of blood lead levels in affected individuals are essential to prevent toxicity.

Metrology is the science of measurement. It deals with the establishment of units of measurement, their standards, and the development of new measuring methods and instruments. Metrology ensures accuracy and consistency in measurements across scientific research, engineering, manufacturing, and trade. It is broadly classified into three types — scientific metrology, industrial metrology, and legal metrology.

Other options (for context)

• Weather: The study of weather and atmosphere is called Meteorology, not Metrology.
• Distance: Measuring distance is just one aspect of metrology, not its entire scope.
• None of these: Incorrect — metrology specifically refers to measurement science.

Explanation: Mucor is not a biofertilizer; it is a saprophytic fungus that grows on decaying organic matter. Biofertilizers are living microorganisms that enhance soil fertility by fixing atmospheric nitrogen or solubilizing nutrients. Examples include Rhizobium, Azotobacter, and Anabaena.

Explanation: The tusks of elephants are actually modified second upper incisors. They are made primarily of dentine, commonly known as ivory, and covered by a thin layer of enamel at the tip (which wears off with time). Tusks grow continuously throughout the elephant’s life and are used for digging, lifting objects, stripping bark, defense, and dominance displays.

Note: Both male and female African elephants have tusks, whereas in Asian elephants, tusks are generally present only in males.

Composition: Tusks are composed of dentine (ivory) with fine concentric growth lines, and they grow from sockets in the skull.

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.