The Rule of Thumb for Neglecting the Change in the Formal

The rule of thumb for neglecting the change in the formal concentration of a weak acid or a weak base,x,during an equilibrium problem is valid:

A) if the change in x is greater than 1% of the formal concentration.
B) if the change in x is less than 0.1% of the formal concentration.
C) if the change in x is equal to 0.1% of the formal concentration.
D) if the change in x is less than 10% of the formal concentration.
E) if the change in x is less than 1% of the formal concentration.