Answer:
A.
Step-by-step explanation:
To check if a graph is a function, use the vertical line test. Use a ruler or straight edge, or simply imagine a vertical line. Move the line across the graph. The line should only intersect the graph at one point at any time.
B isn't a function because the line intersects the graph at two places (when x = 0).
C isn't a function because the line intersects the graph at two places (between x = -1 and x = 1).
D isn't a function because the line intersects the graph at two places (when x < -1 and when x > 1).
Only A is a function.
