Final answer:
The A-V node and Purkinje fibers can both serve as secondary pacemakers for the heart if the primary pacemaker (SA node) fails or is blocked.
Step-by-step explanation:
The secondary pacemakers of the heart include both the A-V node (atrioventricular node) and the Purkinje fibers. In the normal sequence of cardiac electrical conduction, the sinoatrial (SA) node is the primary pacemaker, initiating each heartbeat. If the SA node fails or its signal is blocked, the A-V node can take over, though at a slower rate. The Purkinje fibers can also act as pacemakers if the above nodes fail; however, they will have an even slower rate than the A-V node. Therefore, the correct answer to which might serve as a secondary pacemaker for the heart is 'c. both A-V node and Purkinje fibers.'