Sound speed order: gases < liquids < solids

Steps:

• Speed of sound v = sqrt(B/ρ), where B is bulk modulus (elasticity) and ρ is density.
• Gases have low B and low ρ, resulting in slowest speed (~343 m/s in air).
• Liquids have higher B than gases despite higher ρ, yielding intermediate speed (~1500 m/s in water).
• Solids have highest B and moderate ρ, giving fastest speed (~5000 m/s in steel).

Why C is correct:

• Option C lists Q_g (gas/air), Q_l (liquid), Q_s (solid), matching the increasing speed order per the wave speed formula.

Why the others are wrong:

• A starts with liquid (too fast) before gas.
• B starts with liquid and ends with gas (reverses gas-liquid order).
• D starts with solid (fastest) instead of slowest gas.

Final answer: C