In the Reaction Below, What Does the Symbol ⇌ Mean

In the reaction below, what does the symbol ⇌ mean? OH- + NH₄+ ⇌ H₂O + NH₃

A) It means that the forward and backward reactions are happening at the same time.
B) It means that the reaction cannot decide which way to go.
C) It means that the forward reaction does not progress.
D) It means that the backward reaction happens as fast as the forward reaction and so the proton is not transferred at all.
E) none of the above