In Calculating the X-Ray Energy Levels the Effective Charge of the Nucleus

Multiple Choice

In calculating the x-ray energy levels the effective charge of the nucleus is taken to be Z - b, where Z is the atomic number.The parameter b enters because:

A) an electron is removed from the inner shell
B) a proton is removed from the nucleus
C) the quantum mechanical force between two charges is less than the classical force
D) the nucleus is screened by electrons
E) the Pauli exclusion principle must be obeyed