If a cell of constant emf produces the same amount of heat during the same time in two independent resistors $$R_1$$ and $$R_2$$ when they are separately connected across the terminals of the cell, one after the other. The internal resistance of the cell is

A

$$\frac{R_1 + R_2}{2}$$

B

$$\frac{R_1 - R_2}{2}$$

C

$$\frac{\sqrt{R_1^2 + R_2^2}}{2}$$

D

$$\sqrt{R_1 R_2}$$