A tapering bar (diameters of end sections being \[{{d}_{1}}\] and\[{{d}_{2}}\]) and a bar of uniform cross-section 'd' have the same length and are subjected to the same axial pull. Both the bars will have the same extension if 'd' is equal to

• A. \[\frac{{{d}_{2}}+{{d}_{2}}}{2}\]
• B. \[\sqrt{{{d}_{1}}{{d}_{2}}}\]
• C. \[\sqrt{\frac{{{d}_{1}}{{d}_{2}}}{2}}\]
• D. \[\sqrt{\frac{{{d}_{2}}+{{d}_{2}}}{2}}\]