小白有问题请教 tpo18-2

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tpo阅读18-2
第二段中标黄部分
Another flaw of the tiredness theory is that yawning does not raise alertness or physiological activity, as the theory would predict. When researchers measured the heart rate, muscle tension and skin conductance of people before, during, and after yawning, they did detect some changes in skin conductance following yawning, indicating a slight increase in physiological activity. However, similar changes occurred when the subjects were asked simply to open their mouths or to breathe deeply. Yawning did nothing special to their state of physiological activity.Experiments have also cast serious doubt on the belief that yawning is triggered by a drop in blood oxygen or a rise in blood carbon dioxide.  Volunteers were told to think about yawning while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide.  If the theory was correct, breathing air with extra carbon dioxide should have triggered yawing, while breathing pure oxygen should have suppressed yawning.  In fact, neither condition made any difference to the frequency of yawning, which remained constant at about 24 yawns per hour.  Another experiment demonstrated that physical exercise, which was sufficiently vigorous to double the rate of breathing, had no effect on the frequency of yawning. Again, the implication is that yawning has little of nothing to do with oxygen.
标黄部分their代指谁啊，怎么翻译

aaaaaaaaaaaaaaaaaUID：3798958

救命啊，

TitandustUID：3783749

their 是指people，或者subjects（实验对象）
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