A 0050 M Solution of the Weak Acid HA Has [H₃O⁺]

A 0.050 M solution of the weak acid HA has [H₃O⁺] = 3.77 × 10⁻⁴ M. What is the K_a for the acid?

A) 7.5 × 10 ⁻³ M
B) 2.8 × 10 ⁻⁶ M
C) 7.0 × 10 ⁻⁷ M
D) 7.0 × 10 ⁻⁸ M
E) 2.6 × 10 ⁻¹¹ M