18.) Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Thus option B is the correct answer. 19) Nicolas Copernicus developed the heliocentric model in the 16th century, which led to the believe that the sun is the center of the Universe, with the planets (including Earth) and stars orbiting it. 20.) A gas giant is a large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune.
