What is the reference point for the Solar System?

The Sun

Explanation:

A good reference point to use when determining the motion of planets in the solar system is the sun because it is in the center of the universe and all of the planets revolve around it. Because the sun is considered stationary, it allows us to see if the planets are in motion or not.

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.