Down a Column in the Periodic Table, the Ionization Energy $\ell$

Down a column in the periodic table, the ionization energy ________

A) increases because the n quantum number changes.
B) increases because the $\ell$ quantum number changes.
C) decreases because the outer electrons are further from the nucleus.
D) increases because the atomic number (nuclear charge) increases.
E) changes little because the outer electrons all are in the same outer shell.