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: Dissolved oxygen (DO) is the amount of oxygen available in water for aquatic organisms to breathe. For the healthy survival of most aquatic life, the DO level should be between 4 and 6 mg/L. Levels below 3 mg/L can lead to stress or suffocation, especially for fish and aerobic microorganisms. Higher DO levels are typical of clean, cold, and well-aerated waters, while polluted or stagnant waters often have lower oxygen levels due to organic decay.

Factors Affecting DO Levels:

• Temperature (higher temperature = lower DO)
• Organic pollution and bacterial activity
• Photosynthesis by aquatic plants and algae
• Water turbulence and aeration

When iron rusts, its weight increases. This is because rusting is a chemical reaction between iron, oxygen, and moisture from the air, forming hydrated iron(III) oxide (Fe₂O₃·xH₂O). During this process, oxygen atoms from the air combine with the iron, adding extra mass to the metal surface in the form of rust. However, while the weight increases, the structural strength of the iron decreases due to corrosion.

Other options (for context)

• Decreases: Incorrect — mass increases due to the addition of oxygen.
• Remains the same: Incorrect — oxygen from air chemically adds to iron, increasing total mass.
• First increases and then decreases: Not applicable — in normal rusting, weight continues to increase until the surface flakes off.

The pale yellow colour of urine is due to the presence of the pigment Urochrome. Urochrome is a waste product formed from the breakdown of hemoglobin in red blood cells. It is produced when hemoglobin is metabolized into urobilin and then excreted through urine. The intensity of the yellow color can vary depending on hydration levels — darker urine indicates dehydration, while pale urine indicates proper hydration.

Other options (for context)

• Urophyll: No such pigment exists in human physiology.
• Chlorophyll: Green pigment found in plants, not in urine.
• Chloroplast: Cell organelle in plants where photosynthesis occurs, not a pigment.

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: Ampicillin is a semi-synthetic derivative of penicillin and belongs to the β-lactam group of antibiotics. It inhibits bacterial cell wall synthesis and is effective against a wide range of gram-positive and gram-negative bacteria. Therefore, it is classified as a broad-spectrum antibiotic. Paracetamol is not an antibiotic, and penicillin is a narrow-spectrum antibiotic.

Explanation: The biosphere is the global sum of all ecosystems — it encompasses every region of Earth where living organisms are found. It includes portions of the lithosphere (land), hydrosphere (water), and atmosphere (air) that support life. The biosphere extends from deep ocean trenches to the lower layers of the atmosphere where microorganisms can survive.