Final answer:
The Sun Belt came into play in the mid-20th century, specifically after the widespread adoption of air-conditioning in the 1950s.
Step-by-step explanation:
In the mid-20th century, the Sun Belt came into play. The Sun Belt refers to the region of the United States that stretches across the southern and southwestern states, characterized by warm climate and rapid population growth. The popularity of the Sun Belt for service-sector jobs emerged after the widespread adoption of air-conditioning in the 1950s.