Which of the contributing structures of the resonance below is more stable?

![enter image description here][1]

I'm watching a video lecture by a professor of my college where he puts this question to the class. The class unanimously says **B**. At first I thought they were wrong, but then the professor agreed with them. 

Is this true? The reason he gave was that oxygen's octet would be complete in **B**, hence it is more stable. But I don't understand this.

1) Isn't the octet complete in **A** too? It has 2 covalent bonds - *4 electrons (2 from sharing)* and 2 lone pairs. Adding up to 8.  
2) Oxygen being more electronegative than carbon, would pull the electrons towards itself, leading me to think that it would be more comfortable with a lone pair rather than another bond. Also, won't it prefer to lose out that '+' charge which the bonding's burdened onto it? 

Am I wrong?

Another thing which made me believe that Prof. may have made a mistake while talking about the more stable compound, is that when he added Br$^-$ to the compound, he added it to the compound A. And don't you continue reactions with the most stable products?
  [1]: https://i.sstatic.net/6Y13D.png