The `f`-block elements of the periodic table contains those element in which
A. only 4f orbitals are progressively filled in `6^(th)` period.
B. only 5f orbitals are progressively filled in `7^(th)` period
C. 4f and 5f orbitals are progressively filled in `6^(th)` and `7^(th)` periods respectively
D. none
