Final answer:
When using a manometer, if the mercury level is pushed up due to the gas pressure being higher than the atmospheric pressure, the pressure difference should be added to the atmospheric pressure.
Step-by-step explanation:
In a manometer, if the mercury is pushed up towards the atmospheric pressure, you should add the pressure difference to determine the pressure of the gas in the bulb. This is because the pressure exerted by the gas is the sum of the atmospheric pressure and the difference in height of the mercury columns. Therefore, if the gas pressure is higher than the atmospheric pressure, the correct action would be to add the difference in mercury level to the atmospheric pressure (Option A).