Literal equation:
we know that
literal equation is equation of more one variable
for example: 
And we can solve such equations only if we are given two such equation
because it has two variables
One variable equation:
such equation has single variables
for example:
we don't need any other equation
we can solve for variables from single equation itself
because it has only one variable that is x
Given: Volume of brains in cubic centimeters.
Because it is talking about volume it is something you can ordered, difference can be found and are meaningful. There is also a natural starting zero point because something can have 0 volume. Like an empty cup. It has no water in it for volume.
Answer:
D. The ratio level of measurement is most appropriate because the data can be ordered, differences can be found and are meaningful, and there is a natural starting zero point
