Answer the following question given this paragraph: 

Human organs are organized into organ systems. An organ system is a group of organs that work together to carry out a complex function. Each organ of the system does part of the overall job. For example, the heart is an organ in the circulatory system. The circulatory system also includes the blood vessels and blood. There are many different human organ systems. Figure below shows six of them and gives their functions.


Q: The heart is an organ in which system?

 Choices:

- nervous system

- immune system

- respiratory system

- circulatory system

A:
circulatory system
