Three resistors having resistances $r_{1}, r_{2}$ and $r_{3}$ are connected as shown in the given circuit. The ratio $\frac{i_{3}}{i_{1}}$ of currents in terms of resistances used in the circuit is :

• A
  $\frac{r_{1}}{r_{2}+r_{3}}$
• B
  $\frac{r_{2}}{r_{2}+r_{3}}$
• C
  $\frac{r_{1}}{r_{1}+r_{2}}$
• D
  $\frac{r_{2}}{r_{1}+r_{3}}$