Plz help me with this question

Cars rust because when metal comes into contact with water or some sort of water form, it will make the surface wet. If the metal is wet for too long, it will start to soak up the water, therefore making it rusty. The salt will also rust a car. It does this because the mix of carbon dioxide and oxygen in the salt will eat away at the car, making it rust faster. Hope this helps!

a:

by the combination of an unlimited number of elements