The lack of which gas is the general cause of yawning?

The struggle for breath, say when you are under water, is not lack of oxygen but build up carbon dioxide. Similar but not the same

so both answers could be right, but maybe not !

this question made me yawn (contagious)

One of my medications years ago had a 'yawning' side effect, someone be talking to me and all of a sudden I'm yawning and yawning, I'd get weird looks like I'm being rude and bored with them, till I end up explaining my yawning fits! 🤷🏼‍♀️🤦🏼‍♀️😅

bloggs, you never “need more” CO2, so there is no ambiguity in this question.

bloggs, there are 4 answers to choose from. Unless you mean between needing more oxygen or less carbon dioxide