TOEFL Listening: TPO-TOEFL阅读TPO - 293VCF3UQ02XL0U9N$

The study of yawning behavior discussed in paragraph 4 supports which of the following conclusions? A. Yawning is associated with an expectation of increased physical activity. B. Yawning occurs more frequently when people are asked to record their yawning. C. People tend to yawn about fifteen minutes before they become tired or bored D. Mental or physical stress tends to make people yawn