Concept:The mercury column height depends on the pressure difference between the atmosphere and the space above the mercury.Explanation:In a barometer, the mercury column balances the atmospheric pressure.When air is introduced into the vacuum above the mercury, the air molecules exert extra downward pressure on the mercury surface in the tube.This extra pressure increases the pressure above the mercury.As a result, the mercury column is pushed down, so its height decreases.Answer:B. increase the pressure above the mercury.