If $n$ and $l$ are principal and azimuthal quantum numbers respectively,then the expression for calculating the total number of electrons in any energy level is:

• A
  $\sum\limits_{l = 0}^{l = n} {2(2l + 1)} $
• B
  $\sum\limits_{l = 1}^{l = n - 1} {2(2l + 1)} $
• C
  $\sum\limits_{l = 0}^{l = n + 1} {2(2l + 1)} $
• D
  $\sum\limits_{l = 0}^{l = n - 1} {2(2l + 1)} $