Find the odd one out and why:

a)iron, Aluminium, Copper, Sodium.

b)Carbon, Sulphur, Phosphorus,Bromine.

c)Helium, Neon, Argon,Hydrogen.

a) sodium, since it reacts very vigorously and fast withe the atmosphere and the other metals don't.

b) bromine, since the other non-metals are solid and this one is liquid.

c) helium, since it is only used to float balls or balloons and the othergases are used in lamps.