Which of the following describes a difference between galaxies and solar systems?
a) Galaxies contain only moons, while solar systems contain planets.
b) Gravity holds solar systems in orbits but does not exist between stars in galaxies.
c) A galaxy is a collection of millions of stars, while a solar system is a star and the planets around it.
d) It's easier to use astronomical units to measure distances in the galaxy but use light years for measurements in a solar system.