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 2.

The correct option to fill in blank number (2) is "when".

Let's analyze the sentence structure and the context of the passage to understand why this option is correct:
The sentence reads: "Some people believe that we yawn (2) ________ we do not have enough oxygen in our bodies."

Here, the sentence explains a condition or a specific time at which people believe yawning occurs. The conjunction "when" is used to introduce a clause describing the time or circumstance under which an event happens (in this case, yawning occurring at the time we lack oxygen). Therefore, "we yawn when we do not have enough oxygen" is grammatically correct and logically coherent.

Let's look at why the other options are less appropriate:
- "as": While "as" can mean "because" or "while", in this specific context, "when" is the most natural fit to indicate the condition or situation.
- "why": "Why" indicates a reason, but using it here ("we yawn why we do not have enough oxygen") is grammatically incorrect.
- "while": "While" usually refers to a continuous action happening simultaneously with another, which does not fit the cause-and-effect condition described here as precisely as "when".