单项选择题

Questions 17—20 are based on the following monologue about yawn. You now have 20 seconds to Read Questions 17—20. What physiological reason for yawning is mentioned

A．To exercise the jaw muscles.
B．To eliminate fatigue.
C．To get greater strength for attacking.
D．To gain more oxygen.

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单项选择题

单项选择题

Questions 17—20 are based on the following monologue about yawn. You now have 20 seconds to Read Questions 17—20. What physiological reason for yawning is mentioned 您的浏览器不支持 audio 元素。