Two cars $A$ and $B$ start from a point at the same time in a straight line and their positions are represented by $R_{A}(t) = at + bt^2$ and $R_{B}(t) = xt - t^2$. At what time do the cars have the same velocity?

• A
  $\frac{x-a}{2(b+1)}$
• B
  $\frac{x+a}{2(b-1)}$
• C
  $\frac{x-a}{(b+1)}$
• D
  $\frac{x+a}{(b-1)}$