The popularization of expanded the communications and sports industries.
radios
talkies
the model t
airplanes

Radios

Explanation:

Talkies might help you to communicate, but not so much to spread something around massively. Airplanes are not a communication method, neither is the T Model. Radios transmitted sound to many, many people and sometimes, a game would be occurring and the Radio would have someone to watch and narrate it for you. It did make sports grow.

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i think a judge or jury, as they preside over the court and is the central figure in a trial.

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food chain i would say only because there is competition on getting food and survival