What will be equivalent resistance of two resistors $$R_1$$ and $$R_2$$ when connected in series?

A

$$R_1 - R_2$$

B

$$R_1 + R_2$$

C

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

D

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