Answer the following question given this paragraph: 

Animals not only have specialized cells. Most animals also have tissues and organs. In many animals, organs form organ systems, such as a nervous system. Higher levels of organization allow animals to perform many complex functions. What can animals do that most other living things cannot? Here are some examples. All of them are illustrated in Figure below .


Q: Groups of related organs form what?

 Choices:

- community systems

- maturation systems

- artificial systems

- organ systems

A:
organ systems
