What usually happens to radio waves with frequencies below the LUF?
They are refracted back to Earth
They pass through the ionosphere
They are attenuated before reaching the destination✓الإجابة الصحيحة
They are refracted and trapped in the ionosphere to circle Earth
الشرح
Below the LUF, a wave is still bent back by the ionosphere, but absorption in its lowest layer removes so much energy on the upward and downward paths that the returned signal is too weak to be usable. The limiting factor is loss, not a failure of refraction.