Hydrostatic pressure difference Steps:

• Pressure due to sea water at depth h is P = ρ g h (gauge pressure, density ρ constant).
• At 30 m: p = ρ g × 30.
• At 60 m: P₂ = ρ g × 60 = 2p.
• Difference: P₂ - p = ρ g × (60 - 30) = ρ g × 30 = p.

Why C is correct:

• By the hydrostatic pressure formula, ΔP = ρ g Δh; here Δh = 30 m equals the initial depth, so ΔP = p.

Why the others are wrong:

• A. 0: Depths differ, so pressures differ by ρ g Δh ≠ 0.
• B. 0.5p: Requires Δh = 15 m, but actual Δh = 30 m.
• D. 2p: Requires Δh = 60 m, but actual Δh = 30 m.

Final answer: C