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This is not an externality because it was the original intended use of air-conditioning. The passage mentions that Carrier invented air-conditioning to ensure cool, dry air for factories with low-humidity requirements.
The other options are externalities:
Option b: This is an unintended consequence. CFCs were initially used in refrigeration and air-conditioning, but their long-term environmental impact (ozone depletion) was not anticipated.
Option c: The Jacquard loom was originally a mechanical device for weaving patterns in fabric, but it led to the development of programmable machines, which had far-reaching effects on modern computing, which was also an unintended consequence of the loom's invention.
Option d: The passage states that the phonograph was initially designed for dictation but was adapted for music recording, which was an unintended consequence.
Hence, option a is the correct answer.