Two cells of emf $$2E$$ and $$E$$ with internal resistance $$r_1$$ and $$r_2$$ respectively are connected in series to an external resistor $$R$$ (see figure). The value of $$R$$, at which the potential difference across the terminals of the first cell becomes zero is:

A

$$r_1 + r_2$$

B

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

C

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

D

$$r_1 - r_2$$