Two cars P and Q start from a point at the same time in a straight line and their positions 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

$$\frac{a+f}{2(b-1)}$$

B

$$\frac{a+f}{2(1+b)}$$

C

$$\frac{f-a}{2(1+b)}$$

D

$$\frac{a-f}{1+b}$$