Answer
See the explanation below.
Work Step by Step
A function is considered to be written as $f(x,y,z)$ with three variables $x,y$ in a set of domain D of real numbers. A contour map is the best method for visualizing a function of two variables. It consists of level surfaces such as $f(x,y,z)=p$ (p=a constant term). These are horizontal traces of the graph of the function which shows the projection onto the $xyz$ plane.
