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