Mercury boils at ${367}^0\mathrm{C}$. However, mercury thermometers are made such that they can measure temperature up to ${500}^0\mathrm{C}$. This is done by

• A

  maintaining vacuum above mercury column in the stem of the thermometer

• B

  filling nitrogen gas at high pressure above the mercury column

• C

  filling nitrogen gas at low pressure above the mercury level

• D

  filling oxygen gas at high pressure above the mercury column