Answer:
three-phase
Step-by-step explanation:
2-phase, 3-phase, and 6-phase systems are in use. The most common of these is 3-phase.
A modern industrial plant most likely uses a 3-phase system.
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Additional comment
Serious research began on 6-phase power transmission systems in 1973. They have been shown to increase power transfer, and to reduce corona, magnetic fields, and audio noise compared to 3-phase systems. However, they are not in widespread industrial use.