A quantity of air is enclosed in a glass bulb that is placed in water and connected to a U-tube filled with mercury. The densities of the water and the mercury are $\rho_w$ and $\rho_m$ respectively. The water surface is exposed to the atmosphere, where the atmospheric pressure is $P$. The acceleration of free fall is $g$. What is the pressure of the air in the glass bulb?
- A$P + g\rho_w h_w + g\rho_m h_m$
- B$P + g\rho_w h_w - g\rho_m h_m$
- C$g\rho_w h_w + g\rho_m h_m$
- D$g\rho_w h_w - g\rho_m h_m$