The graph shows the number of paintballs, y, a machine launches in x seconds: A graph titled Rate of Launch is shown. The x-axis label is Time in seconds, and the x-axis values are from 0 to 20 in increments of 4 for each grid line. The y-axis label is Number of Balls, and the y-axis values from 0 to 160 in increments of 32 for each grid line. A line is shown connecting points on ordered pair 4, 32 and 8, 64 and 12, 96 and 16, 128.
Which expression can be used to calculate the rate per second at which the machine launches the balls? (1 point)

fraction 32 over 4
fraction 4 over 32
fraction 128 over 4
fraction 4 over 128