The Amount of Interstellar Matter Present in Our Galaxy Is

Multiple Choice

The amount of interstellar matter present in our Galaxy is always changing. Which of the following processes is NOT a major contributor to that change?

A) gas from outside the Galaxy falls into the Milky Way because of gravity
B) the powerful explosions that end the lives of massive stars drive material out of the Galaxy
C) interstellar matter in dense clouds collapses into stars and is taken "out of circulation"
D) some atoms of gas combine in dusty clouds to make more complex molecules
E) actually, none of the above is a major contributor to the change