The position of an object moving along the x-axis is given by x = a + bt^2, where a = 8.5 m and b = 2.5 m/s^2 and t is measured in seconds. The average velocity of the object between t = 2 s and t = 4 s is:

A) 20 m/s
B) 5 m/s
C) 10 m/s
D) 15 m/s