A J-shaped tube closed at the short end traps $20\mathrm{cm}$ of air when the mercury levels in the two limbs are equal and atmospheric pressure is $76\mathrm{cm}\mathrm{Hg}$. More mercury is poured into the open limb until the trapped air column is $16\mathrm{cm}$ long. At constant temperature, the difference between the mercury levels in the two limbs is now

1. A$19\mathrm{cm}$
2. B$95\mathrm{cm}$
3. C$4\mathrm{cm}$
4. D$15\mathrm{cm}$