Why do people yawn? No one seems to know the exact (1)________. Some people believe that we yawn (2) ________ we do not have enough oxygen in our bodies. In (3) ________ words, we yawn to breathe in (4) ________ oxygen. People also believe that yawning is associated (5) ________ boredom.

Select the most appropriate option to fill in blank number 4.

The correct answer is more.

Step-by-step Explanation:

1. We are asked to fill in blank number 4 in the sentence: "In (3) ________ words, we yawn to breathe in (4) ________ oxygen."

2. Let's analyze the context leading up to this sentence. The previous sentence states: "Some people believe that we yawn (2) ________ we do not have enough oxygen in our bodies."

3. The phrase "In other words" in blank (3) signals that the next clause is a rephrasing or explanation of the idea that we lack sufficient oxygen. If our bodies "do not have enough oxygen," the natural response/purpose of yawning would be to bring in a larger quantity of oxygen to compensate for the shortage.

4. Now let's evaluate the given options for blank (4):
- less: "to breathe in less oxygen" would contradict the idea of not having enough oxygen.
- little: "to breathe in little oxygen" would also mean taking in a small amount, which does not help with a lack of oxygen.
- much: While "much" indicates a large amount, it is not grammatically natural here when describing the comparative increase needed to remedy a deficiency.
- more: "to breathe in more oxygen" logically means inhaling an additional or greater amount of oxygen than before to make up for the lack of it. This fits the context perfectly.

5. Therefore, the most appropriate option to fill in blank number 4 is more.