The correct answer is transistors. • Second-generation computers (1955-1964) were made of transistors. Explanation & Key Takeaways: • Vacuum tube – an electronic device that controls the flow of electrons in a vacuum. It is used as a switch, amplifier, or display screen in many older model radios, televisions, computers, etc. • Transistor – an electronic component that can be used as an amplifier or as a switch. It is used to control the flow of electricity in radios, televisions, computers, etc. • Integrated circuit (IC) – a small electronic circuit printed on a chip (usually made of silicon) that contains many its own circuit elements (e.g. transistors, diodes, resistors, etc.). • Microprocessor – an electronic component held on an integrated circuit that contains a computer’s central processing unit (CPU) and other associated circuits. Important Notes: Five Generations of Computers +--------------------------+-------------------------+ | Generations of computers | Evolving hardware | +--------------------------+-------------------------+ | First-generation | Vacuum tube | | Second generation | Transistor | | Third generation | Integrated circuit | | Fourth generation | Microprocessor | | Fifth-generation | Artificial intelligence | +--------------------------+-------------------------+