The figure above shows the graph of f ', the derivative of the function f on the closed interval 0 ≤ x ≤ 8. The graph of f ' has horizontal tangent lines at x = 2, x = 4, and x = 6. The function f is twice differentiable.
Part A: Find the x-coordinate of each of the points of inflection on the graph of f. Give a reason for your answer.

Part B: At what value of x does f attain its absolute maximum value on the closed interval 0 ≤ x ≤ 8? Show the analysis that leads to your answer.

Part C: For what values of x is the graph of f concave upward? Justify your answer.