The Major Advantages of the 21-Cm Radio Emission from Hydrogen

Multiple Choice

The major advantages of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of the Milky Way Galaxy are that

A) it is relatively easily absorbed by hydrogen gas in the Milky Way, so measurements are not confused by emission of this radiation from other galaxies beyond the Milky Way. It originates only from cold hydrogen gas and can be used to map this important component.
B) radio waves easily penetrate the Milky Way dust and gas, and they are a very narrow-wavelength line emission; thus its Doppler shift can be used to measure gas motions.
C) Doppler shift of this narrow-wavelength line emission is caused by the temperature of the hot hydrogen gas and therefore can be used to measure the distribution and temperature of this important component of the Milky Way.
D) this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas, hence its ability to be used to map the distribution of hot hydrogen gas.