Hydroxyapatite, Which Is a Component of Tooth Enamel, Is Very $\rightleftarrows$

Hydroxyapatite, which is a component of tooth enamel, is very insoluble in water. For the following reaction, K_sp = 2.3 *10^-59.
Ca₅(PO₄) ₃(OH) (s) $\rightleftarrows$ Ca₅(PO₄) ₃⁺(aq) + OH^-(aq)
What is the molar concentration of Ca₅(PO₄) ₃⁺ when the pH of the mouth is 4.0?

A) 2.3 * 10^-49 M
B) 2.3 * 10^-59 M
C) 2.3* 10^-55 M
D) 2.3 *10^-63 M
E) 0, teeth don't dissolve!