Determine whether the following statements are true and give an explanation or counterexample.

a. For linear functions, the slope of any secant line always equals the slope of any tangent line.

b. The slope of the secant line passing through the points P and Q is less than the slope of the tangent line at P.

c. Consider the graph of the parabola f(x) = x². For x > 0 and h > 0, the secant line through (x, f(x)) and (x + h, f(x + h)) always has a greater slope than the tangent line at (x, f(x)).