Choose the word or phrase - A, B, C, or D - that best fits the blank space in the following passage.

Astronauts must be tied to something to prevent them floating around an bumping into equipment (11)__________ they sleep. They may sleep in their seats or in upright lockers or (12)__________ tie themselves to the wall in a sleeping bag. Sometimes they forget to strap down and end up floating around inside the ship, much to the amusement of their fellow crewmembers. On some missions all the crew sleep (13)__________ the same time but during others, they may work round the clock in two shifts. As their ship is traveling (14)__________ fast, they see a sunrise every 90 minutes. Many astronauts find it a difficult way to keep their body clock running smoothly and do not get a lot of sleep. Besides that some of them complain of flashes across their eyelids as they are (15)__________ to sleep caused by cosmic rays zipping through the ship. So although it might seem nice to have a weightless night's sleep it is not always possible in reality.

 

Astronauts must be tied to something to prevent them floating around an bumping into equipment (11)__________ they sleep.