阅读下列短文，从每题所给的四个选项（A、B、C和D）中选出最佳选项。
Bored with your life? Dreaming of something different? When I feel like that I look up and wonder what life would be like 400km above my head. That's where the International Space Station orbits the Earth, with six astronauts living and working on board, for months at a time.
How do they sleep? They spend the night floating in a sleeping bag inside a small cubicle on the ceiling. American astronaut Sunita Williams explains: "It's like a little phone booth but it's pretty comfy and it doesn't matter if I turn over and sleep upside down. I don't have any sensation in my head that tells me I'm upside down."
Brushing your teeth in a place where you can't have a tap or a sink can be a challenge. Can you imagine the mess that running water would make in zero gravity? Canadian astronaut Chris Hadfield uses a straw to put a big blob of water from a sealed bag onto an ordinary toothbrush and adds a little toothpaste - which he has to swallow when he's done.
Daily exercise is essential. The lack of gravity makes bones more brittle and muscles lose strength – so astronauts are encouraged to work out for at least two hours a day.
The role of astronauts in the International Space Station is to act as lab technicians for scientists back on Earth. So they spend their time maintaining their environment and performing and monitoring experiments in a confined space about the size of a Boeing 747. Almost every task is carefully planned by mission control - although most astronauts spend their first days losing stuff until they get used to sticking everything they use to the walls with Velcro, duct tape or clips.
One of their most valued perks is the view from "the office", dominated by that gigantic blue ball down there, sitting in the darkness of space. Wow! Absolutely breathtaking!
2X +2H2O。则X的化学式是（ ）