Choose the correct statement (s) about electronic configuration of lanthanides:
A. all the tripositive ions of the series are of the form `4f^(n)` (n=1 to 14 with increasing atomic number)
B. E.C. of Gd `= [Xe] 4f^(7) 5d^(1) 6s^(2)`
C. E.C. of `Gd^(2+) = [Xe] 4f^(7) 5d^(1)`
D. E.C. of `Ce = [Xe] 4f^(1) 5d^(1)6s^(2)`
