What unit is used to measure the distance between galaxies
a. light years
b. miles
c. kilometers
d. years

A.light years

Explanation:

Astronomical distances, such as the distance between stars or galaxies, are too large. So measuring these distances in the units of measure that we use here on earth would be very impractical, besides they would be very large numbers and difficult to imagine such distances.

For these reasons to measure distances in space it is common to use light years, which contrary to what the name indicates, is not a measure of time but of distance: a light year is the distance that light travels in a year.

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

it is the most commonly accepted one