A capillary tube of uniform bore, sealed at one end, has a $10\mathrm{cm}$ mercury thread trapping an air column that is $20\mathrm{cm}$ long when the tube is horizontal. Atmospheric pressure is $75\mathrm{cm}\mathrm{Hg}$. When the tube is held vertical with the open end up, the length of the air column becomes (constant temperature)

1. A$17.6\mathrm{cm}$
2. B$23.1\mathrm{cm}$
3. C$20\mathrm{cm}$
4. D$15\mathrm{cm}$