A pressure gauge fitted to a closed tank shows a reading of −20kPa. Which statement best describes the pressure inside the tank?
(a) Its absolute pressure is 20kPa absolute (i.e. 20kPa above vacuum).
(b) Its absolute pressure is 20kPa lower than atmospheric pressure (approximately 81kPa absolute).
(c) Its absolute pressure is 20kPa higher than atmospheric pressure (approximately 121kPa absolute).
(d) The tank is at a perfect vacuum, 0Pa absolute pressure.