Concept:Across a period, the trend in first ionization energy is mainly due to increasing positive charge of the nucleus.Explanation:Moving left to right across a period, the nuclear charge increases because more protons are added to the nucleus.Electrons are added to the same principal energy level, so the screening effect remains roughly constant.Thus, the effective nuclear charge on the outermost electrons increases, making them harder to remove and raising the first ionization energy.Answer:B. increase in nuclear charge