Two sources of equal emf are connected to an external resistance $R$. The internal resistances of the two sources are $R_1$ and $R_2 (R_2 > R_1)$. If the potential difference across the source having internal resistance $R_2$, is zero, then :

Question

( A ) $R=R_2-R_1$
( B ) $R = \frac{R_1R_2}{(R_2 - R_1)}$
( C ) $R = \frac{R_2 \times (R_1+R_2)}{(R_2-R_1)}$
( D ) $R = \frac{R_1R_2}{(R_1+R_2)}$