Bromine vapour is heavier than air. even so it's spreads upwards in the experiment above. Why?

Bromine is heavier than air in terms of its atomic weight. Look at the periodic table bromine is lower than oxygen, nitrogen, etc. bromine in its gaseous phase will still float in air. This is due to the fact that gaseous bromine takes up a wide volume of space while having little mass.

i think the answer would be b