Neutral Hydrogen Is Most Obvious in the Electromagnetic Spectrum

Multiple Choice

Neutral hydrogen is most obvious in the electromagnetic spectrum:

A) in yellow light of the Sun's continuum.
B) at 656.3 nm, hydrogen alpha red.
C) at 21 cm in the radio region.
D) at 121.3 nm, the Lyman alpha ultraviolet line.
E) Neutral hydrogen has yet to be detected in space.