Consider a Sample of Water in a Closed Container

Consider a sample of water in a closed container. When the reaction has reached equilibrium what can we say about any specific water molecule?

A) If it was present in the liquid it will always remain in the liquid phase.
B) If it was present in the vapor it will always remain in the vapor phase.
C) It will sometimes be in the liquid phase and sometimes in the vapor phase.
D) Both a and b are true.