Boric Acid Frequently Is Used as an Eyewash to Treat $\times$

Boric acid frequently is used as an eyewash to treat eye infections. The pH of a 0.050 M solution of boric acid is 5.28. What is the value of the boric acid ionization constant, K_a?

A) 5.25 $\times$ 10^-6
B) 5.51 $\times$ 10^-10
C) 5.43 $\times$ 10^-8
D) 5.79 $\times$ 10^-4
E) 5.33 $\times$ 10^-12