One Reason Electronegativity Increases from Left to Right Across a Period

One reason electronegativity increases from left to right across a period is because of _______.

A) increased shielding of valence electrons by the core electrons.
B) an increase in the number of protons in the nucleus.
C) a decrease in the effective nuclear charge Z_eff) .
D) an increase in the shielding of core electrons by the valence electrons.
E) a decrease in the ionization potential.