17. 주어진 문장이 들어갈 위치로 가장 적절한 것은?

Some of these ailments are short-lived; others may be long-lasting.
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For centuries, humans have looked up at the sky and wondered what exists beyond the realm of our planet. ( ① ) Ancient astronomers examined the night sky hoping to learn more about the universe. More recently, some movies explored the possibility of sustaining human life in outer space, while other films have questioned whether extraterrestrial life forms may have visited our planet. ( ② ) Since astronaut Yuri Gagarin became the first man to travel in space in 1961, scientists have researched what conditions are like beyond the Earth's atmosphere, and what effects space travel has on the human body. ( ③ ) Although most astronauts do not spend more than a few months in space, many experience physiological and psychological problems when they return to the Earth. ( ④ ) More than two-thirds of all astronauts suffer from motion sickness while traveling in space. In the gravity-free environment, the body cannot differentiate up from down. The body's internal balance system sends confusing signals to the brain, which can result in nausea lasting as long as a few days.

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