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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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It is easy to get the feeling that everyone except you is living a full, rich and busy life in

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。