According to Bohr's theory the radius of electron in an orbit described by principal quantum number n and atomic number Z is proportional to
(a) ${\mathrm{Z}}^{2}{\mathrm{n}}^{2}$            (b) $\frac{{\mathrm{Z}}^{2}}{{\mathrm{n}}^{2}}$
(c) $\frac{{\mathrm{Z}}^{2}}{\mathrm{n}}$              (d) $\frac{{\mathrm{n}}^{2}}{\mathrm{Z}}$