If the pressure between the visceral pleura of the lungs and the parietal pleura of the thoracic cage is −6 mmHg, then

D: The intra-pleural pressure must be negative (=below atmospheric pressure) if the lungs are to expand as the chest wall expands. It oscillates between about −2 mmHg (−270 Pa) when inhalation is about to begin, and about −6 mmHg (−800 Pa) when inhalation has ended and exhalation is about to occur. Hence a value of ~6 mmHg indicates that inhalation is occurring.