Consider the following statements about the respiratory system: I. The trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchus. II. Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli. III. The pleura is a double-layered membrane that covers the lungs, with pleural fluid reducing friction on the lung surface. Which of the above statements are correct?