Read the given expression. X = number of protons - number of core electrons Which of the following explains the identity of X and its trends across a period? (5 points) O X is the effective nuclear charge, and it remains constant across a period.
O X is the screening constant, and it remains constant across a period.
O X is the effective nuclear charge, and it increases across a period.
O X is the screening constant, and it increases across a period.​