Answer the following question given this paragraph: 

Mollusks have a circulatory system with one or two hearts that pump blood. The heart is a muscular organ that pumps blood through the circulatory system when its muscles contract. The circulatory system may be open or closed, depending on the species.


Q: The heart is a muscular organ that pumps blood through which system?

 Choices:

- endocrine

- pulmonary

- nervous

- circulatory

A:
circulatory
