Please choose the correct answer to the question: An artificial pacemaker is a device that regulates the beat of the heart by an electrical stimulus. Some early pacemakers were powered by batteries and were worn externally with wires passing through the skin to the surface of the heart. Modern pacemakers are now much smaller and are implanted surgically. Which most likely has increased the lifespan and effectiveness of artificial pacemakers?

Answer1: the development of thinner wires Answer2: the invention of lithium batteries Answer3: mass production of pacemaker parts Answer4: reducing the cost of producing pacemakers

Answer format: answer1/answer2/answer3/answer4
Answer: answer2