题目内容：
阅读材料，回答题。Another thing an astronaut has to learn about is eating in space. Food is weightless, just as men are.
Food for space has to be packed in special ways. Some of it goes into tubes that a man can squeeze into his mouth. Bite sized cookies are packed in plastic.
There is a good reason for covering each bite. The plastic keeps pieces of food from travelling in the spaceship. On the earth very small pieces of food would simply fall to the floor. But gravity doesn’t pull them to the floor when they are out of the plastic in a spaceship. They move here and there and can get into a man’s eyes or into the spaceship’s instruments. If any of the instruments is blocked, the astronauts may have trouble getting safely home.
As astronauts travel on longer space trips, he must take time to sleep. An astronaut can fit himself to his seat with a kind of seat belt. Or, if he wants to, he can sleep in a sleeping bag which is fixed in place under his seat. But be careful he must put his hands under the belt when he goes to sleep. This is because he is really afraid that he might touch one of controls that isn’t supposed to be touched until later.
Why would astronauts cover each bite of food in space? A．Because small pieces of food would fall down to the floor.
B．Because weightless pieces of food might make trouble when they travel around.
C．Because they haven’t enough food for longer trips.
D．Because astronauts don’t want to waste food.
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