In a titration, the unknown acid H₂SO₄ reacts with 1.0 M NaOH. The initial burette reading is 1.3 mL, and the final burette reading is 19.6 mL. What is the concentration of the acid (H₂SO₄)?
a. 0.64 M
b. 1.0 M
c. 0.32 M
d. 0.16 M