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Initial voyages into space introduced questions scientists had never before considered. Could an astronaut swallow food in zero

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问题     Initial voyages into space introduced questions scientists had never before considered. Could an astronaut swallow food in zero gravity? To keep things simple, astronauts on the Project Mercury ate foods squeezed out of tubes. It was like serving them baby food in a toothpaste container.
    But these early tube meals were flavorless, and astronauts dropped too many pounds. "We know that astronauts have lost weight in every American and Russian manned flight," wrote NASA scientists Malcolm Smith in 1969. "We don’t know why." Feeding people in space was not as easy as it looked.
    Floating around in space isn’t as relaxing as it might sound. Astronauts expend a lot of energy and endure extreme stresses on their bodies. Their dietary requirements are therefore different from those of their gravity-bound counterparts on Earth. For example, they need extra calcium to compensate for bone loss. ’A low-salt diet helps slow the process, but there are no refrigerators in space, and salt is often used to help preserve foods," says Vickie Kloeris of NASA. "We have to be very careful of that."
    By the Apollo missions, NASA had developed a nutritionally balanced menu with a wide variety of options. Of course, all the items were freeze-dried or heat- treated to kill bacteria, and they didn’t look like regular food.
    Today, the most elaborate outer-space meals are consumed in the International Space Station (ISS), where astronauts enjoy everything from steak to chocolate cake. The ISS is a joint venture between the U.S. and Russia, and diplomatic guidelines dictate the percentage of food an astronaut must eat from each country. NASA’s food laboratory has 185 different menu items, Russia offers around 100, and when Japan sent up its first crew member in 2008, about 30 dishes came with him. Due to dietary restrictions and storage issues, astronauts still can’t eat whatever they want whenever they feel like it.
    In 2008, NASA astronaut and ISS crew member Sandra Magnus became the first person to try to cook a meal in space. It took her over an hour to cook onions and garlic in the space station’s food warmer, but she managed to create a truly delicious dish: grilled tuna (金枪鱼) in a lemon-garlic-ginger sauce---eaten from a bag, of course.
The passage mainly introduces ______.
选项 A、the variety of food options in space
B、the dietary need of astronauts in space
C、the problems of living in the space station
D、the improvement of food offered in space
答案D
解析
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