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

When we measure the narrow line emissions of hydrogen at 21-cm radio wavelength along a particular line of sight through the disk of our Galaxy,we can tell the distances to different hydrogen clouds because

A) clouds at different distances have different Doppler shifts due to the rotation of the Galaxy.
B) the farther away the gas cloud,the greater the delay in the arrival time of the 21-cm emission.
C) clouds that are farther away have smaller angular sizes.
D) the emission is weaker from clouds that are farther away.