A reactant concentration falls from 1.2 to 0.9 mol L⁻¹ in 10 s. What is its average rate of disappearance?

A. 0.3 mol L⁻¹ s⁻¹ B. 0.03 mol L⁻¹ s⁻¹ C. 0.003 mol L⁻¹ s⁻¹ D. 0.21 mol L⁻¹ s⁻¹