Which of the following electronic technologies is the key technology for making the first-generation electronic computer?

The first generation of electronic computers was built around vacuum tubes as the key switching technology.

Vacuum tubes made these machines very large, power-hungry, and prone to heat and failure.

Transistors defined the second generation, being smaller and more reliable than tubes.

Integrated circuits defined the third generation, packing many transistors onto a chip.

Dual-core CPUs are a much later development within microprocessor-based computers.

ENIAC and similar early machines relied on thousands of vacuum tubes.