The Big Bang Theory explains the origin of the Universe. According to this theory, the universe began about 13.8 billion years ago from an extremely hot and dense state that expanded rapidly in all directions. This expansion continues even today, leading to the formation of galaxies, stars, planets, and other cosmic structures. The discovery of the cosmic microwave background radiation provides strong evidence supporting the Big Bang Theory.

Other options (for context)

• Origin of Sun: Explained by the Nebular Hypothesis, not the Big Bang Theory.
• Laws of Chemistry: Studied under physical and inorganic chemistry, unrelated to cosmology.
• Laws of History: Concern human civilizations, not the universe’s formation.

Explanation: Sound travels fastest in steel because it is a solid material where the particles are packed very closely together. Sound waves need a medium to travel, and their speed depends on how quickly vibrations can pass from one particle to another. In solids, particles are tightly bound, so vibrations are transmitted more efficiently. In contrast, sound travels slower in liquids and slowest in gases. It cannot travel through a vacuum because there are no particles to carry the vibrations.

Explanation: When a bus starts suddenly, the passengers tend to lean backward due to the inertia of rest. Inertia is the tendency of a body to resist any change in its state of motion or rest. The lower part of the passenger’s body moves forward with the bus as it starts, but the upper part of the body tends to remain at rest momentarily. This makes the passenger appear to fall backward relative to the bus. It is a clear example of Newton’s First Law of Motion.

Explanation: According to the Bureau of Indian Standards (BIS) – IS 10500:2012, the acceptable limit of total hardness in drinking water, expressed as CaCO₃, is 200 mg/l. The permissible limit in the absence of an alternate source is up to 600 mg/l. Excess hardness can cause scaling and affect the taste of water.

Explanation: Marijuana is a psychoactive drug obtained from the female inflorescence (flowering tops) of the Cannabis sativa plant. It contains the active compound tetrahydrocannabinol (THC), which affects the central nervous system and alters mood and perception.

Explanation: Wind energy is a clean and renewable source of energy that does not cause environmental pollution. It is generated by converting the kinetic energy of moving air into mechanical or electrical energy using wind turbines. Unlike fossil fuels such as coal or mineral oil, wind energy does not release carbon dioxide, sulfur dioxide, or other greenhouse gases. It is, therefore, a pollution-free and sustainable source of power, widely used for electricity generation in wind farms.

The normal hearing range of human beings is from 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz are called infrasonic sounds, while those above 20,000 Hz are known as ultrasonic sounds, both of which are inaudible to humans. With age or prolonged exposure to loud noise, the upper hearing limit tends to decrease, reducing sensitivity to high-frequency sounds.

Other options (for context)

• 30 to 30,000 Hz: Incorrect — exceeds the upper audible limit for humans.
• 50 to 50,000 Hz: Incorrect — animals like dogs and bats can hear this range, not humans.
• 40 to 40,000 Hz: Incorrect — the human ear cannot detect frequencies that high.

Explanation: Vitamin K is produced by gut bacteria (Escherichia coli) in the large intestine. It plays a vital role in the synthesis of prothrombin, a protein essential for normal blood clotting. Although some of it is obtained from food (like leafy vegetables), intestinal bacteria supply a significant portion of the body’s requirement.