The first ionization energy of gallium (Ga, 578.8 kJ/mol) is greater than that of aluminum (Al, 577.5 kJ/mol). Which is the best explanation for this?
a) Gallium has a higher effective nuclear charge than aluminum.
b) Gallium has more electron shielding compared to aluminum.
c) Gallium has a larger atomic radius than aluminum.
d) Gallium has fewer valence electrons than aluminum.