In diagram 1, a manometer filled with mercury (Hg) is linked to a gas tap. The tap is off, so the mercury levels on the two sides are equal. When the gas supply is switched on, the mercury in the tube joined to the supply drops by $40\,\text{mm}$ and the mercury in the tube open to the atmosphere rises by $40\,\text{mm}$, as shown in diagram 2. What pressure does the gas in the gas supply have?
- A$40\,\text{mm Hg}$ above atmospheric pressure
- B$40\,\text{mm Hg}$ below atmospheric pressure
- C$80\,\text{mm Hg}$ above atmospheric pressure
- D$80\,\text{mm Hg}$ below atmospheric pressure