Which of the following factors may be regarded as the main cause of lanthanide contraction ?

Question

( A ) Poorer shielding of 5d electron by 4f electrons
( B ) Effective shielding of one of 4f electrons by another in the sub-shell
( C ) Greater shielding of 5d electron by 4f electrons
( D ) Poor shielding of one of 4f electron by another in the sub-shell