Formic Acid, Which Is a Component of Insect Venom, Has

Formic acid, which is a component of insect venom, has a K_a = 1.8 × 10⁻⁴. What is the [H₃O⁺] in a solution that is initially 0.10 M formic acid, HCOOH?

A) 4.2 × 10 ⁻³ M
B) 8.4 × 10 ⁻³ M
C) 1.8 × 10 ⁻⁴ M
D) 1.8 × 10 ⁻⁵ M
E) 1.8 × 10 ⁻⁶ M