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.

SCIENCE FLYING IN THE FACE OF GRAVITY

It looked just like another aircraft from the outside. The pilot told his young passengers that it was built in 1964, a Boeing KC-135 refueling tanker, based on the 707. 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 from floor to ceiling; it looked a bit like a lunatic asylum. There were almost no windows, but lights along the padded walls eerily 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 apprehension.

From 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 Boeing's flight resembled that of an enormous bird which had lost its reason, shooting upwards towards the heavens before hurting towards Earth. The intention 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 engine cut out and we became weightless. Everything became confused, and left or right, up or down no longer had any meaning. After 10 seconds of free-fall descent, 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. After two hours of going up and down in the plane doing experiments, the predominant feeling was one of exhilaration rather than nausea. Most of the students thought it was an unforgettable experience and one they would be keen to repeat.

What does the word it in the last paragraph refer to