Which of the following factor may be regarded as the main cause of lanthanide contraction?
A. Poor shielding of `4` f-electrons in compare to other electrons in the sub-shell.
B. Effective shielding of one of the `4`f-electrons by another in the sub shell.
C. Poorer shielding of `5` d electrons by `4`f-electrons.
D. Greater shielding of `5f` d-electrons by `4` f-electrons.