All of the following are reasons why astronomers have difficulty looking at distant stars except:
a. some stars are so far away, even the best telescopes are not powerful enough.
b. they don't understand how redshift affects stars.
c. there is a lot of gas and dust in the atmosphere that distorts the light coming from stars.
d. there is a lot of light pollution on earth.

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.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

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