Determine the concentration of hydronium ions for a 25∘C solution with a pOH of 3.5.

A) 1.0 × 10⁻³ M
B) 1.0 × 10⁻⁴ M
C) 1.0 × 10⁻⁵ M
D) 1.0 × 10⁻⁶ M