What is ullage?

• A. The volume of propellant stored inside a tank neglecting its empty volume
• B. The extra volume of gas above the propellant in a tank
• C. The ratio of the volume of extra gas to that of the propellant after 50% of the propellant has been consumed
• D. The ratio of the volume of the propellant to that of the extra gas after 50% of the propellant has been consumed