The electronic configuration of Eu (Atomic No. 63), Gd (Atomic No. 64) and Tb (Atomic No. 65) are :
A. [Xe]4f⁷6s²,[Xe]4f⁸6s² and [Xe]4f⁸5d¹6s²
B. [Xe]⁴f⁷5d¹6s²,[Xe]4f⁷5d¹6s² and [Xe]4f⁹6s²
C. [Xe]4f⁶5d¹6s²,[Xe]4f⁷5d¹6s² and [Xe]4f⁸5d¹6s²
D. [Xe]4f⁷6s²,[Xe]4f⁷5d¹6s² and [Xe]4f⁹6s²
