The correct answer is C: The atmosphere refracts radio waves slightly. The radio horizon for VHF and UHF signals is more distant than the visual horizon because the atmosphere refracts (bends) radio waves slightly. This bending extends the effective range of radio signals beyond what you can see. Radio waves are refracted by the atmosphere due to changes in air density and moisture content with altitude. This refraction bends the radio waves slightly downward, following Earth's curvature more than light does. This extends the radio horizon beyond the visual horizon - you might be able to communicate with stations you can't see. The effect is more pronounced at lower frequencies and under certain atmospheric conditions. This is why your radio can 'see' farther than your eyes can.