Gadolinium (atomic number $= 64$) is a member of the $4f$ series. Its electronic configuration in the $+3$ oxidation state is $[Xe] 4f^7$. What is the ground state electronic configuration of gadolinium?

• A
  $[Xe] 4f^{10}$
• B
  $[Xe] 4f^8 6s^2$
• C
  $[Xe] 4f^7 5d^3$
• D
  $[Xe] 4f^7 5d^1 6s^2$