At What Point in the Following Titration Curve for a Weak

At what point in the following titration curve for a weak acid being titrated with a strong base is the pH equal to the pK_a of the acid? The x-axis scale goes from 0.0 mL to 20.0 mL. The sharp rise is at 10.0 mL.

A) 0.0 mL
B) 5.0 mL
C) 9.0 mL
D) 10.0 mL
E) 18.0 mL