In Order for a Cold Atom of Hydrogen to Emit

In order for a cold atom of hydrogen to emit a 21-cm wave, it must first be in a slightly higher energy state. What event usually "kicks" the hydrogen atom up to this higher state?

A) waves of ultraviolet energy from hot stars pass by a cold gas cloud
B) a gas cloud collapses under its own gravity until it is much smaller than it originally was
C) gas atoms within the cloud collide
D) hydrogen is ionized by other waves
E) some alcohol molecules within the cloud lead to a wild party