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. `sum_(l=1)^(l=n)2(2l+1)`

B. `sum_(l=1)^(l=n-1)2(2l+1)`

C. `sum_(l=0)^(l=n+1)2(2l+1)`

D. `sum_(l=0)^(l=n-1)2(2l+1)`