• The spectra from Croesus is red shifted which indicates it is moving away from Earth.
• The thin spectra lines from Croesus also indicate that it is not rotating.
• The centre of spectral lines from Dromus are blue shifted which indicates it is moving towards Earth. 
• Dromus’ spectral lines are wide and blurry (simultaneously red and blue shifting), which indicates that it is rotating.

By elimination:

• The star’s surface temperature can be found by determining the peak wavelength from its spectrum then using Wein’s Law. (Eliminate B)
• The atmospheric composition of the star can be found by matching its spectra to the known spectra of gases. (Eliminate C)
• Comparing the spectral lines of the star with those of corresponding compounds on Earth, the degree of red shift can be calculated. Applying the doppler effect, the recessional velocity of the star can be determined. (Eliminate D)

`=>A`

• Spectra provide information about the linear velocity of a star relative to Earth. A shift towards higher wavelengths indicates that a star is moving away from Earth. A shift towards lower wavelengths indicates a star is moving towards earth.
• Darkened, broader spectral lines indicate that a star has a high density due to high pressures in the star’s atmosphere. Narrower spectral lines indicate a lower density star.
• Blurring and broadening of the edges of spectral lines indicate a star has a high rotational velocity due to simultaneous red and blue doppler shifting. More defined, narrower spectral lines indicate a star with lower rotational velocity.