Passage One 　　Have you ever dreamed of visiting a planet in the Milk Way While the trip sounds exciting, it would take years and years to reach your destination. So in the future, bedtime for astronauts may be more than a few hours of regular shut-eye. They would have to sleep for years. 　　European researchers are now conducting hibernation experiments. The study may help them understand whether humans could ever sleep through the years it would take for a space flight to distant planets. “If there was an effective technology, it could make deep-space travel a reality,” said Mark Ayre of the European Space Agency last month. 　　What seems like science fiction is not completely unlikely. Researchers have been able to use chemicals to put living cells into a sleep-like state where they don’t age. They have now moved on to small, non-hibernating mammals like rats. The results will be out by the end of 2009. 　　A major challenge is the fact that cells can be very simple systems, whereas body organs are far more complex. 　　"It’s like moving from a simple Apple computer to a supercomputer," said Marco Biggiogera, a hibernation researcher at Italy’s University of Pavia. 　　Just like bears and frogs, the hibernation of human beings would cause a person’s metabolism (新陈代谢) to lower so they would need less energy. 　　Medical research, however, is just half of a space flight hibernation system. 　　There is the challenge of designing a suitable protective shelter. Such a shelter would provide the proper environment for hibernation, such as the proper temperature. It would also have to monitor (监控) life functions and serve the physiological needs of the hibernator. 　　According to Ayre, the six-person Human Outer Planets Exploration Mission to Jupiter’s moon (木星的卫星) Callisto, could be an opportunity to use human hibernation. The mission aims to send six humans on a five-year flight to Callisto, where they will spend 30 days, in 2045.

What’s the best title for the passage

A．Six humans to fly to Callisto
B．Human hibernation improves health
C．Space travel attracts people
D．Deep sleep for deep space travel