Which statement best describes the overall periodic trend in first ionization energy for main-group elements in the periodic table?
a) It generally decreases from left to right across a period and decreases from top to bottom down a group.
b) It generally increases from left to right across a period and decreases from top to bottom down a group.
c) It generally decreases from left to right across a period and increases from top to bottom down a group.
d) It generally increases from left to right across a period and increases from top to bottom down a group.