What adaptations in the exchange surfaces lining the lungs and the intestines of animals have increased their effectiveness?the surface area for exchange has increased due to folds and branchesthe volume of the cells in these linings has increasedthe thickness of the membranes in these linings has increasedthe metabolic rate of the cells in these linings has decreasedthe number of cell layers in these linings has increased

The lung is an important vital organ of humans and many animals. It is an internal organ necessary for gaseous exchange and breathing. The gaseous exchange occurring in the lungs is responsible for the transmission of oxygen to the whole body. Intestines are the main part of the digestive system. Intestines absorb the nutrition from the food and transfer it to the various tissues and cells through blood. This part of the digestive system is followed by the stomach. In the lungs, the serous membrane called pleura folds onto itself to produce a double-layered structure. It forms a thin cavity between these layers called the pleural cavity. The pleural cavity thus formed helps in the efficient working of the lungs at the time of breathing. It helps in the transmission of chest wall movements to the lungs especially in case of heavy breathing. Alveoli of lungs also develop folds known as secondary septa that increase the surface area of alveoli and enhance the gas exchange efficiency. The small intestine of many animals including humans consists of many folds caused by mucus membrane known as plicae circulares. These folds contain villi and microvilli that are small projections that further increase the surface area of absorption and hence the efficiency of absorption of food. Hence the first option is correct.