Two wires of resistance R1 and R2 have temperature coefficient of resistance \[{{\alpha }_{1}}\text{ and }{{\alpha }_{2}}\], respectively. These are joined in series. The effective temperature coefficient of resistance is [MP PET 2003]

• A.            \[\frac{{{\alpha }_{1}}+{{\alpha }_{2}}}{2}\]               
• B.            \[\sqrt{{{\alpha }_{1}}{{\alpha }_{2}}}\]
• C.            \[\frac{{{\alpha }_{1}}{{R}_{1}}+{{\alpha }_{2}}{{R}_{2}}}{{{R}_{1}}+{{R}_{2}}}\]      
• D.            \[\frac{\sqrt{{{R}_{1}}{{R}_{2}}{{\alpha }_{1}}{{\alpha }_{2}}}}{\sqrt{R_{1}^{2}+R_{2}^{2}}}\]