When You Set a Pot of Tap Water on the Stove

When you set a pot of tap water on the stove to boil,you'll often see bubbles start to form well before boiling temperature is ever reached.Explain this observation.

A) These are dissolved salts heating up and escaping from the water.
B) These bubbles are formed as the surrounding gases from the air dissolve into the water as it is heated.
C) They are very minute pockets of water in the gaseous phase.When they get large enough,the water will boil as this gas escapes.
D) These initial bubbles are the gases that were dissolved in the water coming out of solution.The solubility of gases in water decreases with increasing temperature.