Which of the Following Describes the Mechanism by Which Fluoride

Which of the following describes the mechanism by which fluoride is believed to provide protection against dental caries?

A) Fluoride neutralizes tooth-damaging acid by-products produced by bacteria.
B) Fluoride anions substitute for hydroxyl anions in the hydroxyapatite crystal structure, causing the crystal structure to be harder.
C) Fluoride serves as an activator of an enzyme which functions to replenish the hydroxyapatite crystal structure to keep teeth strong.
D) Fluoride is a natural component of tooth enamel that is easily dissolved and must be replenished to provide protection against tooth decay.