Lanthanoid contraction occurs because
A. the 4f electrons, which are gradually added, create a strong shielding effect
B. the 4f orbitals are greater in size than the 3d and 3f orbitals
C. the 5f orbitals strongly penetrate into the 4f orbitals
D. the poor shielding effect of 4f electrons is coupled with increased attraction between the nucleus and the added electrons.
