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For questions 1-8, read the text below and think of the word which best fits each gap. Use only one word in each gap.
Everybody yawns, but scientists still do not fully understand why. For a long time, yawning was thought to be the body's way 1. getting more oxygen. However, experiments in the 1980s showed that people did not yawn more when they breathed air containing less oxygen, 2. did they yawn less when they were given extra. A more recent theory suggests that yawning helps to cool the brain. Supporters point 3. that people tend to yawn more in cool weather than in very hot weather, when breathing in warm air would do little to cool them down. 4. is even more puzzling is the fact that yawning is contagious. Simply seeing, hearing or even reading 5. someone else yawning can make us do the same. This effect seems to be stronger between friends and family members than between strangers, 6. has led some researchers to link it with empathy. Dogs sometimes yawn after watching their owners do so, too, although it is not yet clear 7. this happens for the same reason. If you have yawned at 8. once while reading this, you are certainly not alone.