If x represents time, the average rate of change of the function f(x) in the first three seconds is ____.
A: -20
B: 20
C: 15
D: -15
![If x represents time the average rate of change of the function fx in the first three seconds is A 20 B 20 C 15 D 15 class=](https://us-static.z-dn.net/files/d28/011276865f6338d0d3e9f8c23344f578.png)