A thin wire carrying a total charge \(Q\) is wound into a flat, tightly packed spiral with inner radius \(a\) and outer radius \(b\). The winding turns are spaced uniformly so that the charge can be modeled as a continuous surface charge distribution with uniform surface charge density \(\sigma\) over the region \(a \le r \le b\). Which of the following expressions correctly gives the electric potential at the center of the spiral relative to infinity?