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 and vertex at 12, negative 36

what is the closest approximate average rate of change for edna's profits from the 12th month to the 18th month?

(a)5.92 dollars per month
(b) 3.75 dollars per month
(c) five dollars per month
(d) nine dollars per month