Nearly 10% of all humming bird species are endangered. A wildlife biologists studying the foraging behavior of an endangered humming bird uses a high speed camera to capture the movement of one of the birds. The graph below shows the vertical position of a humming bird in meters as a function of time (seconds).

Assume the time duration covered by the points labeled A, B, and C is 20 ms.

What is the vertical acceleration at point B (t = 8s)?