Two resistances of values 2Ω and 4Ω made of different materials with temperature coefficients of resistance 3 ppm/$$^\circ C$$ and 6 ppm/$$^\circ C$$ respectively are connected in series. What is the temperature coefficient of resistance of the net resistance?

A

5 ppm/$$^\circ C$$

B

$$\sqrt{18}$$ ppm/$$^\circ C$$

C

9 ppm/$$^\circ C$$

D

4.5 ppm/$$^\circ C$$