I.

For a quadratic function $$f(x) = a*x^2 + b*x + c$$, the average rate of change over an interval $$[x_1, x_2]$$ is given by $$a*(x_1+x_2)+b$$.

II.

The average rate of change of a quadratic function over any interval is constant if $$a = 0$$.

III.

For a quadratic function, the instantaneous rate of change at the midpoint of an interval equals its average rate of change over that interval when the endpoints are symmetric about the vertex.

Which of the statements is/are true?