How much pressure of air does it take to lift 100 ft of water?

• A. 29.9 psi
• B. 6.9 psi
• C. 43.5 psi
• D. 100 psi

Answer: (C)

The key idea is hydrostatic head: to lift water, the air must overcome the pressure created by a 100-foot water column. For water, pressure increases about 0.433 psi for every foot of height. So 100 ft of water requires roughly 100 × 0.433 ≈ 43.3 psi. Rounded, that’s about 43.5 psi, which is why this option is correct.

The other numbers don’t fit because they correspond to different water heights (for example, 29.9 psi would lift only about 69 ft of water, 6.9 psi only about 16 ft, and 100 psi would lift roughly 231 ft).