A mass is suspended at the bottom of two springs in series having stiffness \(10 \mathrm{~N} / \mathrm{mm}\) and \(5 \mathrm{~N} / \mathrm{mm}\). The equivalent spring stiffness of the two springs is nearly.

(a) \(0.3 \mathrm{~N} / \mathrm{mm}\)

(b) \(3.3 \mathrm{~N} / \mathrm{mm}\)

(c) \(5 \mathrm{~N} / \mathrm{mm}\)

(d) \(15 \mathrm{~N} / \mathrm{mm}\).