Which if the following is a requirement that units in a correct formula or equation must always satisfy?
a) each unit in the formula, expressed in si base units, must be the same.
b) all units must be units only of mass, length, or time.
c) the units on both sides of the equal sign, expressed in si base units, must be the same.
d) all masses must be in kilograms, all lengths in meters, and all times in seconds.