单选题 We don't have beds in the space shuttle, but we do have sleeping bags. During the day, when we are working, we leave the bags tied to the wall, out of the way. At bed- time we untie them and take them wherever we've chosen to sleep.
On most space shuttle flights everyone sleeps at the same time. No one has to stay awake to watch over the space plane; the shuttle's computers and the engineers at Mission Control do that. ff anything goes wrong, the computers ring an alarm and the engineers call us on the radio.
On the space shuttle, sleeping-time doesn't mean nighttime. During each ninety- minute orbit the sun "rises" and shines through our windows for about fifty minutes; then it "sets" as our path takes us around the dark side of the Earth. To keep the sun out of our eyes, we wear black sleep masks.
It is surprisingly easy to get comfortable and fall asleep in space. Every astronaut sleeps differently. Some sleep upside down, some right side up. When it's time to sleep, I take my bag, my sleep mask, and my tape player with earphones and float up to the flight deck. Then I crawl into the bag, and float in a sitting position just above a seat, right next to a window. Before I pull the mask down over my eyes, I relax for a while, listening to music and watching the Earth go by beneath me.

单选题 When it's bedtime, astronauts put their sleeping bags ______.

单选题 "Watch over" (Line 2, Para.2) has the closest meaning to ______.

单选题 How long does it take for the space shuttle to go round the Earth?

单选题 According to the passage, in order to get comfortable and fall asleep in space, it is necessary to ______.

单选题 The best title for this passage can be ______.