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.

Graphite allows electric current to pass through it. Although it is a form of carbon, its atoms are arranged in layers with free-moving electrons, making it a good conductor of electricity. This property makes graphite useful in electrodes, batteries, and brushes of electric motors.

• Glass, Rubber, PVC: All are insulators and do not conduct electricity.

Explanation: Acids do not turn red litmus to blue; instead, they turn blue litmus paper red. Acids are sour in taste, corrosive in nature, and have a pH value less than 7. Therefore, the statement that acids turn red litmus to blue is incorrect.

Explanation: The sex of a newborn baby is determined by the chromosome inherited from the father. Humans have 23 pairs of chromosomes — 22 pairs of autosomes and one pair of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The mother always contributes an X chromosome, but the father can contribute either an X or a Y. If the sperm carrying an X chromosome fertilizes the ovum, the child will be a girl (XX); if the sperm carries a Y chromosome, the child will be a boy (XY).

Glaciers usually melt first at the bottom. The pressure at the base of a glacier is very high, which lowers the melting point of ice — a phenomenon known as pressure melting. Additionally, heat from the Earth’s crust and friction caused by glacier movement also contribute to faster melting at the base.

• Top surface: Exposed to air but melts slower due to lower pressure.
• Sides and middle surface: Experience less pressure and friction, so melting is slower.

Explanation: Pathogenic parasites are those that invade the host’s body and cause disease. The presence of these harmful organisms stimulates the host’s immune system to produce antibodies that help neutralize or destroy the pathogens. Examples include Plasmodium (causes malaria) and Trypanosoma (causes sleeping sickness).

Explanation: Natural gas is not a renewable source of energy because it is a fossil fuel formed over millions of years from the remains of plants and animals. Once used, it cannot be replenished quickly. In contrast, geothermal energy, flowing water, and biomass are renewable energy sources that can be naturally regenerated.

Explanation: Mercury is the nearest planet to the Sun. It is the smallest planet in the solar system and completes one revolution around the Sun in just about 88 Earth days. Due to its proximity, Mercury experiences extreme temperature variations between day and night.

Explanation: Uranium-235 (U-235) is the isotope used for the production of atomic energy because it is fissionable. When bombarded with neutrons, U-235 undergoes nuclear fission, releasing a large amount of energy along with additional neutrons, which sustain the chain reaction in nuclear reactors and atomic bombs.