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 following questions from 31 to 37.

For many of us, being able to go to space is a rare and wonderful experience. However, before you can officially join NASA and carry out your first mission in space, the road will definitely not be easy.

To begin with, those who want to pursue a career in space exploration must have a master's degree in a relevant field of science and at least two years of relevant professional experience, which amounts to at least 7 to 8 years of preparation in total. The application process, which takes place every four years, is no less challenging. When 18,300 people applied for the class of 2017, for example, only 12 were accepted, which is less than 1% - so your chances of being accepted is definitely slim.

Even if NASA approves your application, it will take some time before you can actually leave Earth. First, you need to complete two years of basic training. Astronaut trainees must be fluent in Russian so that they will be able to effectively communicate with their colleagues and other experts at the Russian Mission Control Centre, especially during important missions. Furthermore, they also have to learn everything about how spaceships work and the complicated science involved in space stations.

And of course, there is also physical training. Astronauts in training need to earn scuba certification and swim three laps in a pool while wearing a flight suit. They must also undergo military land and water survival training, which is essential for emergencies. Finally, trainees must practise in simulations that allow them to get used to high and low atmospheric pressures, which they may experience when working in space.

The word them in paragraph 4 refers to __________