The tiredness theory of yawning does not seem to explain why yawning occurs. A. Although earlier scientific studies strongly supported the tiredness theory new evidence has cast doubt on these findings. B. Some have proposed that yawning plays a role in the development of the lungs before birth but that it serves no purpose in adults C. New studies, along with anecdotal evidence, have shown that the frequency of yawning increases during extended periods of inactivity. D. Evidence has shown that yawning is almost completely unrelated to the amount of oxygen in the blood and is unrelated to sleep behavior E. Fluids in the lungs of the fetus prevent yawning from occurring which disproves the developmental theory of yawning F. There is some evidence that suggests that yawning prepares the body and mind for a change in activity level