When Astronomers Measure the Narrow Line Emissions of Hydrogen at 21-Cm

Multiple Choice

When astronomers measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy, they can tell the distances to different hydrogen clouds because

A) absorption of extragalactic radiation at this wavelength is greater the farther away the absorber is from the Sun.
B) clouds that are farther away have smaller angular sizes.
C) the emission is weaker from clouds that are farther away.
D) clouds at different distances have different Doppler shifts because of the rotation of the Galaxy.