That the electronic configuration of ytterbium `(Z = 70)` is `4f&(14) 5s^(2)` and of lutetium `(Z) = 71)` is `4 f^(14) 5 d^(1) 6 s^(2)` can be explained on the basis of
A. the extra stability of the half-filled orbitals cones
B. the extra stability of the completely filled orbitals
C. the usual rules for the arrangement of electron in their orbits
D. None of these