Let s(t) be the distance traveled by a car at time t on the interval [a, b]. The average velocity of the car on [a, b] is

A. the slope of the tangent line at x = (a + b) / 2

B. the slope of the secant line through (a, f(a)) and (b, f(b))

C. the slope of the tangent line at x = b

D. the slope of the tangent line at x = a