Hi there!
Which of the following is a unit of mass?
Well, lets start with pound.
A pound will not be a unit of mass, it is a unit of weight because it holds a force of gravity on an object.
A millimeter is not a unit of mass, because it is a type of measure.
A newton is a unit of force, not a unit of mass.
Therefore, the only unit of mass will be Kilogram.
Hope this helped!~
