Two close-coiled helical spring of stiffness \(K_{1}\) and \(\mathrm{K}_{2}\) are connected in series. The combination is equivalent to a single spring of stiffness:

(a) \(\sqrt{K_{1} K_{2}}\)

(b) \(\frac{K_{1} K_{2}}{2}\)

(c) \(K_{1}+K_{2}\)

(d) \(\frac{K_{1} K_{2}}{K_{1}+K_{2}}\)