Two cells having the same emf, are connected in series through an external resistance R. Cells have internal resistance r₁ and r₂ respectively. When the circuit is closed, the potential difference across the first cell is zero. The value of R is:

1. $r_1-r_2$

2. $\frac{r_1+r_2}{2}$

3. $\frac{r_1-r_2}{2}$

4. $r_1+r_2$