Why Is It That We Rely Upon the 21-Cm Line

Multiple Choice

Why is it that we rely upon the 21-cm line associated with the spin flip in hydrogen and not the more familiar hydrogen Balmer lines to gather information from the far side of the galactic plane?

A) The Balmer lines are associated with neutral hydrogen, and most of the hydrogen we need to observe is molecular hydrogen.
B) The Balmer lines are associated with molecular hydrogen, and most of the hydrogen we need to observe is neutral hydrogen.
C) The Balmer lines are in the visible and ultraviolet regions of the spectrum, and these do not penetrate the gas and dust clouds easily.
D) Photons released in the spin flip transition have much more energy than those that come from the Balmer transitions.