If n and `l` are respectively the principal and azimuthal quantum numbers , then the expression for calculating the total number of electrons in any energy level is :
A. `overset(l=n)underset(l=0)sum2(2l+1)`
B. `overset(l=n-1)underset(l=1)sum2(2l+1)`
C. `overset(l=n+1)underset(l=0)2(2l+1)`
D. `overset(l=n-1)underset(l=0)sum2(2l+1)`
