Two sources of equal emf are connected to an external resistance $$R$$. The internal resistance 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

A

$$R = R_2 \times (R_1 + R_2)/(R_2 - R_1)$$

B

$$R = R_2 - R_1$$

C

$$R = R_1 R_2/(R_1 + R_2)$$

D

$$R = R_1 R_2/(R_2 - R_1)$$