✅ Correct Answer: Au

Detailed Explanation: Gold (Au) comes from “aurum,” Latin for gold. Silver is Ag (argentum). Atomic number 79. Used in jewelry & electronics due to corrosion resistance. [Periodic Table Reference]

✅ Correct Answer: Mars

Detailed Explanation: Mars appears red due to iron oxide (rust) on surface. Diameter ~6,779 km. Has Olympus Mons (largest volcano in solar system) & evidence of ancient water flows. Next mission: Mars Sample Return (2030s).

✅ Correct Answer: Carbon Dioxide

Detailed Explanation: Photosynthesis equation: \(6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\). Plants absorb CO₂ through stomata, use chlorophyll to convert sunlight into glucose. Release oxygen—essential for life on Earth.

✅ Correct Answer: Diamond

Detailed Explanation: Diamond scores 10 on Mohs hardness scale (hardest known natural material). Pure carbon with atoms in tetrahedral crystal lattice—strongest covalent bonds. Used in drill bits, saws & 90% of industrial applications.

✅ Correct Answer: Vitamin C

Detailed Explanation: Vitamin C (ascorbic acid) essential for collagen synthesis & antioxidant protection. Deficiency → Scurvy: bleeding gums, joint pain, poor wound healing. Historical cure: James Lind’s 1747 citrus experiment on sailors.

✅ Correct Answer: Gravitational force

Detailed Explanation: Newton’s Universal Gravitation: \(F = G\frac{m_1m_2}{r^2}\). Sun’s gravity pulls planets inward while orbital velocity creates centrifugal force outward → stable elliptical orbits (Kepler’s 1st Law).

✅ Correct Answer: The Sun

Detailed Explanation: Solar constant: ~1,366 W/m² at Earth’s orbit. Energy budget: 30% reflected (clouds/albedo), 20% evaporates water, 5% directly warms atmosphere, 50% warms surface → drives weather, ocean currents, climate.

✅ Correct Answer: Mitochondria

Detailed Explanation: Mitochondria: site of cellular respiration (Glucose + O₂ → 36-38 ATP + CO₂ + H₂O). Double membrane, own circular DNA (endosymbiotic origin from ancient bacteria). “Powerhouse” nickname from ATP production.

✅ Correct Answer: 300,000 km/s

Detailed Explanation: Exact value: \(c = 299,792,458\) m/s (defined constant since 1983). Universal speed limit (Special Relativity). Light from Sun takes 8 min 20 sec to reach Earth. Basis for modern meter definition.

✅ Correct Answer: Nitrogen

Detailed Explanation: Dry air composition: N₂ (78.08%), O₂ (20.95%), Ar (0.93%), CO₂ (0.0407%). Nitrogen cycle: fixation by bacteria → plants → animals → decay. Inert gas prevents excessive oxidation/fire risk.