6) Explain what is really happening when a planet (say, Mars)
executes retrograde motion.
Answer this by describing what you saw in the computer simulation that
showed the planets moving in their orbits together with the appearance of
the sky from Earth.
The crucial point is that a planet executes retrograde motion across
the sky when it passes Earth in its orbit, or when Earth passes it. To give
a really good answer, you might give the background information that the
apparent position of the planet in the sky is determined solely by the
direction you have to look in order to see it, and this direction changes,
relative to the direction you have to look to see some star, as the planet
and the Earth move in their orbits.