A reactant concentration falls from 0.5 to 0.1 mol L⁻¹ in 40 s. What is its average rate of disappearance?

A. 0.1 mol L⁻¹ s⁻¹ B. 0.001 mol L⁻¹ s⁻¹ C. 0.01 mol L⁻¹ s⁻¹ D. 0.015 mol L⁻¹ s⁻¹