Read the following passage and choose the correct answer to each of the questions.

The word "robot" first appeared in a 1921 stage play by Czech writer Karel Capek. In the play, a man makes a machine that can think, which he calls a robot and which ends up killing its owner. In the 1940s, the American science fiction writer, Isaac Asimov, wrote a series of stories about robots and invented the term 'robotics', the science of robots. Meanwhile, in the real world, the first robots were developed by an engineer, Joseph F. Engelberger, and an inventor, George C. Devol. Together they started Unimation, a manufacturing company that produced the first real robot in 1961, called the Unimate. Robots of this type were installed at a General Motors automobile plant and proved to be a success. They worked reliably and saved money for General Motors, so other companies were soon acquiring robots as well.

These industrial robots were nothing like the terrifying creatures that can often be seen in science fiction films. In fact, these robots looked and behaved nothing like humans. They were simply pieces of computer-controlled machines, with metal "arms" or "hands". Since they were made of metal, they could perform certain jobs that were difficult or dangerous for humans, particularly jobs that involve high heat. And since robots were tireless and never got hungry, sleepy, or distracted, they were useful for tasks that would be tiring or boring for humans. Industrial robots have been improved over the years, and today they are used in many factories around the world. Though the use of robots has meant the loss of some jobs, at the same time other jobs have been created in the design, development, and production of the robots.

Outside of industry, robots have also been developed and put into use by governments and scientists in situations where humans might be in danger. For example, they can be sent in to investigate an unexploded bomb or an accident at a nuclear power plant. Researchers also use robots to collect samples of hot rocks or gases in active volcanoes. In space exploration, robots have performed many key tasks where humans could not be present, such as on the surface of Mars. In 2004, two robotic Rovers, small six-wheeled computerized cars, were sent to Mars.

What does the word they in paragraph 2 refer to?