Two cells each of emf $$E$$ and internal resistances $$r_1$$ and $$r_2$$ respectively are connected in series with an external resistance $$R$$. The potential difference between the terminals of the first cell will be zero when $$R$$ is equal to

A

$$\frac{r_1 + r_2}{2}$$

B

$$\sqrt{r_1^2 - r_2^2}$$

C

$$r_1 - r_2$$

D

$$\frac{r_1 r_2}{r_1 + r_2}$$