Answer the following question: 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? A:

OPTIONS: (a). endocrine; (b). pulmonary; (c). nervous; (d). circulatory;
Answer:
(d).