A 0.10 M solution of a weak monoprotic acid has a pH equal to 4.0. The ionization constant ka of the acid is
A. 1 x 10⁻3
B. 1 x 10⁻4
C. 1 x 10⁻7
D. 1 x 10⁻8
E. 1 x 10⁻9