1. Which of the following statements about atomic electronic structure is correct? (A) The effective nuclear charge (Zeff) experienced by a valence electron increases down a group (B) Electrons in $d$ orbitals shield outer s electrons more effectively than inner s electrons (C) The energy of $4s$ orbitals is always lower than that of $3d$ orbitals for all transition-metal atoms (D) For a given shell (same n), $p$ orbitals generally have higher energy than $s$ orbitals due to poorer penetration (E) The Pauli exclusion principle allows two electrons in the same atom to have identical sets of four quantum numbers