Gadolinium belongs to the $4f$ series. Its atomic number is $64$. Which of the following is the correct electronic configuration of gadolinium?

• A
  $[Xe] 4f^9 5s^1$
• B
  $[Xe] 4f^7 5d^1 6s^2$
• C
  $[Xe] 4f^6 5d^2 6s^2$
• D
  $[Xe] 4f^8 6d^2$