Read the following passage and mark the letter A, B, C or D to indicate the correct word or phrase that best fits each of the numbered blanks.

The International Space Station (ISS) is a large spacecraft which was launched in 1998. It orbits the Earth and is the place where astronauts live and conduct research in a microgravity (23)…..

Below is about the day in the life of an astronaut aboard the International Space Station.Astronauts sleep in sleeping bags. The microgravity makes them (24)…… . They have to attach themselves so they don’t float about.

Hair is washed with a “rinseless” shampoo that does not need water. For toothbrushing, a small amount of water is used with toothpaste and then swallowed.

The crew spend their day doing science experiments. They also carry out checks for the maintenance of the station. The Mission Control Centre on Earth warns their operations.

The astronauts eat various foods (25)……fruits, nuts, chicken, beef, seafood, brownies, salt and pepper (in liquid forms), etc. Drinks include coffee, tea, and fruits juice (all packaged). Foods can be heated up or kept cool with special machines.

A popular pastime while orbiting Earth is (26)……looking out of the windows to admire its beauty. In their free time, astronauts exercise, watch movies, play music, read books, play cards, and talk to their families.

How are astronauts trained? Anyone can apply to train as an astronaut as long as they have a bachelor’s degree in science, mathematics or engineering, experience as a jet pilot, and are in good health If they get accepted, there are several phases of training. The first phase requires them to pass a swimming test in a flight suit. They take parabolic flights that produce weightlessness. They also learn about various spaceship systems. In the second phase, they are trained to operate spacecraft systems and deal with emergencies. The training often (27)……in a water tank laboratory so that trainees become familiar with crew activities in simulated microgravity in order to perform spacewalks.

The word "optional" in paragraph 3 is CLOSEST in meaning to