How do we know the total mass of the Milky Way Galaxy that is contained within the

 By applying newtons version of keplers 3rd law (or the equivalent orbital velocity law) to the Suns orbit around the center of the galaxy

D, -8 F

Explanation:

This specific thermometer is in increments of two degrees per minor line and 10 per thicker line. Since the thermometer is showing below zero, the answer cannot be A or B since they are positive. -4 Cannot be the answer since the line addition does not make sense for that. It would be -4 if the thick line increments were 5 degrees instead of ten. Therefore if you count the lines by two from zero down to the thermometer line, you would get -8 Degrees F