The mapping shows a relationship between x and y. Which statement is true of the mapping?
A. y is not a function of x, since the y-value 3 corresponds to two different x-values.
B. y is a function of x, since the y-values do correspond to exactly one x-value.
C. y is not a function of x, since the x-values do not correspond to exactly one y-value.
D. y is a function of x, since the x-values do correspond to exactly one y-value.