A thread of mercury seals air inside a closed tube. The mercury thread measures $100\ \text{mm}$ in length. The tube is positioned as shown. Atmospheric pressure is $770\ \text{mm Hg}$. What is the pressure of the trapped air?
- A$100\ \text{mm Hg}$
- B$670\ \text{mm Hg}$
- C$770\ \text{mm Hg}$
- D$870\ \text{mm Hg}$