Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.Called the ‘Red Planet,’ Mars is roughly half the size of Earth, and one of our closest neighboring planets. Though Mars is the most Earth-like of any other planet, the two are still worlds apart. Living on Mars has been the stuff of science fiction for decades. However, can humans really live on Mars? Will it ever be possible or safe? NASA (the National Aeronautics and Space ...

Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Called the ‘Red Planet,’ Mars is roughly half the size of Earth, and one of our closest neighboring planets. Though Mars is the most Earth-like of any other planet, the two are still worlds apart. Living on Mars has been the stuff of science fiction for decades. However, can humans really live on Mars? Will it ever be possible or safe? NASA (the National Aeronautics and Space Administration) hopes to find out. NASA researchers on Earth are conducting several experiments together with the International Space Station (ISS) to study the health and safety issues that may tell us if life on Mars is possible.

Food and oxygen would be the main necessities for travelers living extended periods on Mars. The need to grow plants, which provide both food and oxygen, would be a key. But the decreased gravity and low atmospheric pressure environment of the planet will stress the plants and make them hard to grow. However, space station crews are growing plants in controlled environments in two of the station’s greenhouses. They take care of the plants, photograph them, and collect samples to be sent back to Earth. Researchers then use the data to develop new techniques that will make it possible to grow plants successfully in space.

Another concern for space travelers is the health hazards posed by the effect of space radiation on humans. A spacecraft traveling to Mars would be exposed to large amounts of radiation. Since human exposure to such intense radiation would mean certain death, the spacecraft used for such travel would have to protect the humans on the inside of the craft from exposure. Researchers are using special machines inside the crew areas of the International Space Station to carefully watch radiation levels. NASA scientists, who have maintained radiation data since the beginning of human space flight, continue to learn about the dangers it poses. Researchers use the station to test materials that could be used in making a spacecraft that could successfully travel to Mars.

Will it ever be safe for humans to live on Mars? It is still too early to say. But thanks to the dedicated researchers of NASA and the results of ISS experiments, we are getting closer to knowing every day.

What does the word “they” in paragraph 2 refer to?