Answer:
<h2>A. Domain.</h2>
Step-by-step explanation:
An xy-plane is formed by two variables, an independent variable, which is x, and a dependent variable, which is y.
The set that defines the independent variable values for the relation, it's called Domain.
The set that defines the dependent variable values is called Range.
Therefore, in this case, the right asnwer is A. Domain, becase the set of all x-values is actually the Domain set of the relation.
