The correct answer is: very slow chemical reaction.
A system at equilibrium may appear to be static, with no net change occurring over time. However, this does not mean that chemical reactions have stopped; rather, the forward and reverse reactions are occurring at the same rate, resulting in no net change.
A very slow chemical reaction can be mistaken for a system at equilibrium, as the rates of the forward and reverse reactions may be so slow that no noticeable change occurs over a long period of time.
In contrast, a very fast chemical reaction would likely result in a noticeable change in a short period of time, making it easier to distinguish from a system at equilibrium.
A single substance, by definition, is not a system at equilibrium, as there are no reactants or products to balance.
Therefore, the correct answer is: very slow chemical reaction.
