The electronic configuration of ytterbium (Z = 70) is $4{\mathrm{f}}^{14}5{\mathrm{s}}^2$ and that of lutetium (Z = 71) is $4{\mathrm{f}}^{14}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$ can be explained on the basis of

the extra stability of the half- filled orbitals 

the extra stability of the completely filled orbitals

the usual rules for the arrangement of electrons in their orbits

None of these