Our heart beats - or contracts - approximately 70 times a minute. Each contraction is started by a small group of cells that creates a wave of electricity. This spreads across our heart and causes the muscle fibres to contract. An electrical discharge that spreads across any membrane is called an action potential. Doctors can use an electrocardiogram (ECG) machine to measure the action potentials that spread across the heart muscle. A healthy heart gives the characteristic shape shown in the diagram below for each heartbeat. Thus, option A is correct.