The graph shows the number of paintballs a machine launches, y, 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 80 in increments of 16 for each grid line. A line is shown connecting points on ordered pair 4, 16 and 8, 32 and 12, 48 and 16, 64.
Which expression can be used to calculate the rate per second at which the machine launches the balls? (5 points)

A: fraction 64 over 4
B: fraction 4 over 64
C: fraction 16 over 4
D: fraction 4 over 16