The traffic flow at a particular intersection is modeled by the function ∫ defined by f(t) =25+6cos ( x/3 ) for 0≤ t≤ 120. What is the average rate of change of the traffic flow over the time interval 30≤ t≤ 40
(A) 0.743
(B) 0.851
(C) 0.935
(D) 1.176
(E) 1.265