Two resistors $$R_{1}$$ and $$R_{2}$$ (in ohms) at temperatures $$T_{1} K_{1}$$ and $$T_{2} K_{2} $$respectively, are connected in series. Their equivalent noise temperature is

A

$$T_{1}+T_{2}$$

B

$$R_{1}T_{1}+R_{2}T_{2}$$

C

$$\frac{(R_{1}T_{1}+R_{2}T_{2})}{(R_{1} R_{2})}$$

D

$$\frac{(R_{1}T_{1}+R_{2}T_{2})}{(R_{1}+ R_{2})}$$