题目内容：
请根据短文内容，回答题。We don’t have beds in the spacecraft，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 bedtime we untie them and take them wherever we’ve chosen to sleep．
On most spacecraft flights everyone sleeps at the same time．No one has to stay awake to watch over the spacecraft；the craft’s computers call us on the radio．
On the spacecraft，sleep-time doesn’t mean nighttime．During each ninety minute orbit(轨道) the sun “rises” and shines through our windows for about fifty minutes．then it “sets”as the spacecraft 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(宇航员) sleeps differently some sleep upside down，some sideways，and 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(漂浮)up to the flight deck(驾驶舱)．Then I get 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 under me．
When the astronauts are working，sleeping bags are fastened________． A．on the wall
B．to their seats
C．onto the flight deck
D．anywhere they like
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