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 36 to 40.

Robots are useful for exploring and working in space. In particular, many robots have been sent to explore Mars. Such robots have usually looked like a box with wheels. Though these robots are useful, by their very nature they are unreliable, extremely expensive, and they break easily. Also, they cannot do very many tasks. Because of these problems, scientists have been developing a new and unusual kind of robot. These new robots move like snakes, so they have been given the name "snakebots." The way a snake is shaped lets it get into very small spaces, like cracks in rocks. It can also push its way below the ground or climb up different kinds of objects, like high rocks and trees. Such abilities account for the usefulness of a robot designed like a snake. A snakebot would be able to do these things, too, making it much more effective than regular robots with wheels, which easily get stuck or fall over. Since they can carry tools, snakebots would be able to work in space, as well. They could, for example, help repair the International Space Station. But how can such a robot shape be made? A snakebot is built like a Chain made of about thirty parts, or modules. Each module is basically the same in that they all have a small computer and a wheel to aid movement. The large computer in the "head" of the snake makes all of the modules in a snakebot work together. The modular design of the snakebot has many advantages. If one module fails, another can be added easily. Snakebot modules can also carry different kinds of tools, as well as cameras. Since each module is actually a robot in itself, one module can work apart from the rest if necessary. That is, all the modules can separate and move on their own, and then later, reconnect back into a larger robot. Researchers are also trying to develop snakebots made of a special kind of plastic that can change its shape using electricity, almost like animal muscles. Snakebots made with this plastic will be very strong and hard to break. Overall, the snakebot design is much simpler than that of common robots. Thus, snakebots will be much less expensive to build. For example, a robot recently sent to Mars cost over a hundred million dollars, whereas snakebots can cost as little as a few hundred dollars. With their versatility and affordability, snakebots seem to be the wave of the future, at least as far as space robots are concerned.

What does the word "they" in the second paragraph refer to ?