Your Instructor Hands You a Closed Flask of Room-Temperature Water

Your instructor hands you a closed flask of room-temperature water. When you hold it, the heat from your bare hands causes the water to boil. Quite impressive! How is this accomplished?

A) The pressure inside the flask is very high, so only a slight increase in temperature is required to bring the water to a boil.
B) Your instructor has tricked you and there must be a liquid, such as alcohol, with a lower boiling point than water.
C) A salt must be dissolved in the water to lower its boiling point.
D) The air in the flask is very low in pressure, so that the heat from your hand (not the pressure from your hand) will produce boiling at this reduced pressure.