We Detect Neutral Gas in the Interstellar Medium by Looking

Multiple Choice

We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when:

A) an electron moves from the n = 1 to n = 2 state in a hydrogen atom
B) an electron is ionized from a hydrogen atom
C) carbon monoxide (CO) gas is excited by stellar radiation
D) the spin of an electron flips and aligns with the spin of a proton in a hydrogen atom
E) an electron combines with a proton to make a hydrogen atom