B. They don't understand how red shift affects stars.
Explanation:
Astronomers face difficulty in observing distant stars because they are so far that they are easily viewed by telescope. Light pollution is major concern in present times . There is very less window present for observation. The dim light from the distance stars also gets distorted due to atmosphere and effects astronomical seeing.
Astronomers are aware about the Doppler effect for light that is the redshift effect on the star light. It is not a difficulty for astronomers.
