A function is a relation in which every input has one output.
Relation 1 is a set of points where each x value has a single y value. function
Relation 2 is an equation of a line. With the exception of vertical lines, all lines are functions. This example is not a vertical line. function
Relation 3 is a graph of an absolute value equation (y=|x|-1). We can use the vertical line test and see that it is a function.
Relation 4 has input two traveling to two outputs (A and B). So it is not a function.
Answer: 4
