The graph below shows the value of edna's profits f(t), in dollars, after t months:

graph of quadratic function f of t having x intercepts at 6, 0 and 18, 0, vertex at 12, negative 36, and passes through point 21, 41.25

what is the closest approximate average rate of change for edna's profits from the 18th month to the 21st month?

a. three dollars per month

b. nine dollars per month

c. 13.75 dollars per month

d. 41.25 dollars per month