If Astronomers Aim a Radio Telescope at a Distant Spiral $\alpha$

Multiple Choice

If astronomers aim a radio telescope at a distant spiral arm of the Milky Way Galaxy, they will probably observe a 21-cm line. If they point a large optical telescope at this same region, they will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal. Why not?

A) Neutral hydrogen is incapable of emitting visible radiation.
B) The H _$\alpha$ line-the primary emission of neutral hydrogen-has a much smaller energy than the 21-cm radio wave and thus is harder to detect.
C) There is little energy in the depths of space to excite visible radiation from neutral hydrogen.
D) The visible light emitted by neutral hydrogen is all absorbed by H II gas regions before reaching the solar system.