The Spreading Yawn

Have you ever wonder why when someone nears you yawn, it triggers you to yawn too? Before that, why do we yawn in the first place? Because we are sleepy? or lack of oxygen? or just because by seeing someone yawning?

There are no scientific prove that what exactly caused the yawning, but i remember there's one time when i was watching this show 'Zheong Mun Yan' by Eric Tsang, he asked the exact same questions to the actor/actresses. He asked them why when we see other ppl yawn we will yawn too. There are 3 options to choose from:-

1. When someone nears you yawn, they took away all the oxygen, due to the lack of oxygen, you yawn too.
2. Because you are sleepy and tired.
3. Because when you see ppl yawning, your brain sent this signal to you telling you that you feel sleepy and tired and thus makes you yawn.

Which one you choose? I picked the first option but the correct answer (according to Eric Tsang) is the third one, something to do with the brain. Did you answer correctly?