The diagram shows an open-end mercury manometer being used to measure the pressure of a flowing gas. The manometer reading is 55 mm. Atmospheric pressure is 755 mm Hg.
Approximately what is the gauge pressure of the flowing gas? (Neglect the difference between the masses of the gases in the manometer arms)
A. -55 mm Hg
B. 810 mm Hg
C. 700 mm Hg
D. 55 mm Hg