Chromium has the electronic configuration $4s^1 3d^5$ rather than $4s^2 3d^4$ because

• A
  $4s$ and $3d$ have the same energy
• B
  $4s$ has a higher energy than $3d$
• C
  $4s^1$ is more stable than $4s^2$
• D
  $4s^1 3d^5$ half-filled is more stable than $4s^2 3d^4$