Final answer:
The airplane and mass-produced automobiles were key developments in transportation that enabled mass tourism to become a reality.
Explanation:
The development in transportation that led to the concept of mass tourism was the invention of the airplane and the mass production of automobiles. These advancements significantly transformed travel possibilities, allowing for quicker and more accessible transportation for a larger number of people. The increase in air travel and the widespread ownership of automobiles played crucial roles in facilitating mass tourism by making travel faster, more convenient, and more affordable for the general population.
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