Which of the following factor is responsible for the pulmonary alveoli to be kept dry under normal circumstances?

Pulmonary capillaries and pulmonary lymphatic system normally maintain a slight negative pressure in the interstitial spaces. Whenever extra fluid appears in the alveoli, it will be simply sucked mechanically into the lung interstitium through the small openings between the alveolar epithelial cells. Then the excess fluid is either carried away through the pulmonary lymphatics or absorbed into the pulmonary capillaries. Thus, under normal conditions, the alveoli are kept dry, except for a small amount of fluid that seeps from the epithelium onto the lining surfaces of the alveoli to keep them moist. Ref: Guyton and Hall Textbook of Medical Physiology By John E. Hall, 2010, Chapter 38.