A、The shortage of water, B、The shortage of food. C、The condition of weightlessness. D、The great pressure outside the spacecraft.

Spacemen have to be specially trained for traveling in space. They should be in very good mental and physical health. Most of them are experienced pilots of very fast aeroplanes.
    All spacemen have to do various space exercises before traveling to space. They have to be strong enough to bear the great pressures they will feel at launching and landing. When traveling in space or orbiting round a planet, spacemen have to become used to the weightless conditions and learn how to walk, eat, drink and sleep in a condition of weightlessness. They train for their space travels in model spacecraft, which are like the actual spacecraft. Spacemen also have to know all about the various parts of their spacecraft. They should be able to repair any part that is damaged while traveling.
    When man goes into space he must take air, food and water. He must also get rid of bad air and waste matter. Food is prepared in a paste form and kept in tubes. Waste is stored in containers and got rid of when the spacecraft reaches the Earth. These problems have been solved for short space trips, but not for long trips.