The graph represents revenue in dollars as a function of greeting cards sold. A coordinate plane showing Greeting Card Revenue, Number of Cards Sold on the x-axis and Revenue in dollars on the y-axis. A line starts at (0. 0) and passes through (2, 8), (4, 16), and ends at (5, 20).

Which equation represents the function shown on the graph?

y = x
y = x
y = 2x
y = 4x