Concept:The fountain experiment shows that certain gases are extremely soluble in water, creating a vacuum that pulls water upward.Explanation:Hydrogen chloride gas (HCl) and ammonia gas (NH3) dissolve very rapidly and completely in water.When either gas dissolves, the pressure inside the apparatus drops sharply.This pressure difference forces water to rush upward, producing the “fountain” effect.Their high solubility, not their density or volatility, makes them suitable for this demonstration.Answer:C. very soluble in water