instruction:
In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.
question:
Question: Expansion of a gas into a vacuum is irreversible because the external pressure is measurably less than the internal pressure of the gas, and no states of what exist? Answer: equilibrium
answer:
Expansion of a gas into a vacuum is irreversible because the external pressure is measurably less than the internal pressure of the gas, and no states of equilibrium exist.

question:
Question: What is the key to the demographic transition? Answer: reduced family size
answer:
Reduced family size is the key to the demographic transition.

question:
Question: What are the two divisions of vascular plants? Answer: seedless and seed
answer:
Seedless and seed are the two divisions of vascular plants.