Which of the following factors may be regarded as the main cause of lanthanide contraction?
A. Greter shielding of 5d electrons by 4f electrons
B. Poorer shielding of 5d electron by 4f electrons
C. Effective shielding of one of 4f electrons by another in the sub-shell
D. Poor shielding of one of 4f electron by another in the sub-shell
