Two cars `P` and `Q` start from a point at the same time in a straight line and their position are represented by `x_p(t) = at + bt^2` and `x_Q (t) = ft - t^2`. At what time do the cars have the same velocity ?
A. `(a - f)/(1 + b)`
B. `(a + f)/(2 (b - 1))`
C. `(a + f)/(2 (1 + b))`
D. `(f - a)/(2 (1 + b))`