Which is true about the pleural sac? it is continuous with the atmosphere and protects the lungs from infection. it is a closed sac surrounding each lung and contains a large volume of surfactant. it is a closed, air-filled space surrounding both lungs that has a negative pressure compared to atmospheric pressure. it is bounded by pleural membranes that are very thin, which allows for abundant gas exchange. it is a closed sac surrounding each lung that contains only a tiny volume of lubricating fluid?