Which of the following statements best defines a function?

• A. A set of ordered pairs where every $y$-value is associated with exactly one $x$-value
• B. A relationship where each input $x$ corresponds to exactly one output $y$
• C. Any equation that can be plotted as a straight or curved line on a coordinate plane
• D. A set of points where the number of $x$-values is equal to the number of $y$-values

Answer: (B)