As the heart beats, blood flows through the body. A heart beats starts when a small group of specialized muscle cells
contract. This group of specialized cells is called the sinoatrial (SA) node. Each contraction started by the SA node then
spreads to the two upper chambers of the heart. Where is the SA node located?
O a. in the left ventricle
O b in the left atrium
O c. in the right atrium
O d. in the right ventricle