The word They” in paragraph 3 refers to

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 18 to 24.
Willis Carrier designed the first air-conditioning unit in 1902, just a year after graduating from Cornell University. At a Brooklyn printing plant, fluctuations in heat and moisture were causing the size of the printing paper to keep changing slightly, making it hard to align different colors. Carrier's invention made it possible to control temperature and humidity levels and so align the colors. The invention also allowed industries such as film, processed food, textiles, and pharmaceuticals to improve the quality of their products.
In 1914, the first air-conditioning device was installed in a private house. However, its size, similar to that of an early computer, meant it took up too much space to come into widespread use, and later models, such as the Weathermaker, which Carrier brought out in the 1920s, cost too much for most people. Cooling for human comfort, rather than industrial need, really took off when three air conditioners were installed in a department store in Detroit, Michigan. People crowded into the shop to experience the new invention. The fashion spread from department stores to cinemas.
To start with, money-conscious employers regarded air conditioning as a luxury. They considered that if they were paying people to work, they should not be paying for them to be comfortable as well. So in the 1940s and 1950s, the air-conditioning manufacturers started putting out a different message about their product.
According to their research, installing air conditioning increased productivity amongst employees. They found that typists increased their output by 24% when transferred from a regular office to a cooled one. Another study into office working conditions, which was carried out in the late 1950s, showed that the majority of companies cited air conditioning as the single most important contributor to efficiency in offices.
However, air conditioning has its critics. Jed Brown, an environmentalist, complains that air conditioning is a factor in global warming. Unfortunately, he adds, because air conditioning leads to higher temperatures, people have to use it even more. However, he admits that it provides a healthier environment for many people in the heat of summer.