Toss Up - Earth And Space - Multiple Choice

Which of the following statements best describes the difference between a galaxy and a nebula?
W) A galaxy consists of stars; a nebula consists of dust and gas.
X) There are two types of nebula, but only one type of galaxy.
Y) A galaxy always emits light; a nebula never emits light.
Z) A galaxy consists of matter; a nebular consists of energy.