单项选择题

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It looked just like another aircraft from the outside. The pilot told his young passengers that it was built in 1964．But appearances were deceptive, and the 13 students from Europe and the USA who boarded the aircraft were in for the flight of their lives.
Inside, the area that normally had seats had become a long white tunnel. Heavily padded (填塞) from floor to ceiling, it looked a bit strange. There were almost no windows, but lights along the padded walls illuminated it. Most of the seats had been taken out, apart from a few at the back, where the young scientists quickly took their places with a look of fear.
For 12 months, science students from across the continents had competed to win a place on the flight at the invitation of the European Space Agency. The challenge had been to suggest imaginative experiments to be conducted in weightless conditions.
For the next two hours, the flight resembled that of an enormous bird which had lost its reason, shooting upwards towards the heavens before rushing .towards Earth. The invention was to achieve weightlessness for a few seconds.
The aircraft took off smoothly enough, but any feelings that I and the young scientists had that we were on anything like a scheduled passenger service were quickly dismissed when the pilot put the plane into a 45-degree climb which lasted around 20 seconds. Then the engines cut out and we became weightless. Everything became confused and left or right, up or down no longer had any meaning. After ten seconds of free-fall descent(下降) the pilot pulled the aircraft out of its nosedive. The return of gravity was less immediate than its loss, but was still sudden enough to ensure that some students came down with a bump.
Each time the pilot cut the engines and we became weightless, a new team conducted its experiment. First it was the Dutch who wanted to discover how it is that eats always land on their feet. Then the German team who conducted a successful experiment on a traditional building method to see if it could be used for building a future space station. The Americans had an idea to create solar sails that could be used by satellites.
After two hours of going up and down in the lane doing their experiments, the predominant feeling was one of excitement rather than sickness. Most of the students thought it was an unforgettable experience and one they would be keen to repeat.

According to the passage, the purpose of being weightless was to （）.

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