Optima Prime is designing a robot quilt for her new grandson. She plans for the robot to have a square face. The amount of fabric that she needs for the face will depend on the area of the face, so Optima decides to model the area of the robot's face mathematically. She knows that the area A of a square with side length X units (which can be inches or centimeters) is modeled by the function A (x)=x^2 square units. WHAT IS THE DOMAIN OF THE FUNCTION A(X) IN THIS UNIT CONTEXT?