The number we need in order to answer the question belongs in the space between the words "is" and "of". You left that blank blank, so there really isn't any question here to answer.
HOWEVER ... the refractive index of a medium can never be less than 1.0 , so we know for sure that <em>choice-a can't be</em> the correct answer.
One side of the wave changes speed before the other side, causing the wave to move
Answer:
Visible light has a shorter wavelength than radio waves
Since its a sphere, the top is seen first because its the tallest part if the ship. If the earth was flat, the whole ship would be seen.