Which of the following electronic configurations is not possible according to Hund's rule?

• A
  $1s^2 \, 2s^2$
• B
  $1s^2 \, 2s^1$
• C
  $1s^2 \, 2s^2 \, 2p_x^1 \, 2p_y^1 \, 2p_z^1$
• D
  $1s^2 \, 2s^2 \, 2p_x^2$