An underwater research beacon emits acoustic waves uniformly in all directions. At a radial distance of 8.0 m from the beacon, the displacement amplitude of the water molecules is 1.8 μm1.8\ \mu\text{m}1.8 μm.
What is the displacement amplitude of the water molecules at a radial distance of 20.0 m from the beacon?
0.29 μm0.29\ \mu\text{m}0.29 μm
0.72 μm0.72\ \mu\text{m}0.72 μm
1.1 μm1.1\ \mu\text{m}1.1 μm
4.5 μm4.5\ \mu\text{m}4.5 μm
