(05.01 LC) 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 25 in increments of 5 for each grid line. The y-axis label is Number of Balls, and the y-axis values from 0 to 150 in increments of 30 for each grid line. A line is shown connecting points on ordered pair 5, 30 and 10, 60 and 15, 90 and 20, 120.
Which expression can be used to calculate the rate per second at which the machine launches the balls? (1 point)

a
fraction 120 over 5

b
fraction 5 over 120

c
fraction 5 over 30

d
fraction 30 over 5