Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

• It is the conventional theory that when people are bored or sleepy, they often experience a drop in blood oxygen levels due to their shallow breathing.
• The conventional theory is that people yawn when bored or sleepy because yawning raises blood oxygen levels, which in turn raises alertness.
• According to conventional theory, yawning is more likely to occur when people are bored or sleepy than when they are alert and breathing deeply.
• Yawning, according to the conventional theory, is caused by boredom or lack of sleep and can be avoided through deeper breathing.

This is a multiple choice.