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.

Q: 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

A: 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.
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Q: Question: What once most common bird in north america became extinct in the 1800s? Answer: the passenger piegon

A: The passenger piegon once most common bird in north america became extinct in the 1800s.
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Q: Question: What is group 16 of the periodic table referred to? Answer: oxygen group

A:
Oxygen group is group 16 of the periodic table referred to.
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