Question

through the miracles of medicine and technology, researchers have had some success with growing and transplanting hollow, relatively simple organs such as tracheas and bladders. building organs like the heart and lungs have proved to be much more difficult. doctors have had luck transplanting hearts and lungs. but science has not been very successful in growing hearts and lungs. why? human hearts and lungs are not like any other animal organs so there is not source for cells and tissues. they are complex organs made of many types of cells and tissues. they contain nerves and scientists cannot grow nerve cells. even if researchers build a heart or lung, there is no guarantee that a persons body will accept it.

Explanation:

Hearts and lungs are complex organs composed of diverse cell and tissue types, which makes growing them a challenge. Other options are either inaccurate (there are animal - derived sources for study in some cases) or focus on aspects like nerve cells or organ acceptance which are not the main reason for the difficulty in growing these organs.

Answer:

They are complex organs made of many types of cells and tissues.