Put in increasing order of propagation speed

• A. Air, Water, Bone
• B. Air, Bone, Water
• C. Bone, Water, Air
• D. Water, Air, Bone

Answer: (B)

Propagation speed in ultrasound depends on how compressible and dense a medium is. The speed v is roughly proportional to the square root of stiffness divided by density (v ≈ sqrt(B/ρ)). Air has very low stiffness and very low density, so sound travels the slowest there. Water has much higher stiffness and density, giving a speed around 1500 m/s. Bone is very stiff and dense, so sound travels the fastest, about 4000 m/s. So the increasing order is air, water, bone.