A、wheat, soybeans, carrot and lettuce B、soybeans, tomatoes, carrot and wheat C、white potatoes, carrot, lettuce and soybeans D、wh

The American Space Agency is continuing experiments that would make possible extended periods of space travel. One of these experiments is aimed at finding ways to grow food in space. Scientists at the Kennedy Space Center in the Southern state of Florida are doing these experiments. They are attempting to grow vegetables that could survive without sunlight and dirt. Their laborator is a large circular contain er. Computers control the levels of light, heat, water, fertilizer, and gases in the laboratory’s atmosphere. The plants are grown in special devices that hold about 1 centimeter of liquid plant food. Plastic sheets cover the roots of the plants. The containers are designed to keep the plants from floating away in the weightless environment of space. Water is collected and reused. Bacteria and fungi help turn plant matter into simple sugars that could be added to the astronaut’s food or fed to fish.
   In the special containers plants can develop rapidly in a short period of time. Astronauts would need only to watch the computers to see if the plants are getting what they need.
   The scientists at the Kennedy Space Center laboratory have been able to produce small kinds of wheat, soybeans, lettuce, and white potatoes. Universities around the country are testing other small size crops, such as carrot, tomatoes and rice in special growing containers.
   Scientists also are attempting to genetically design some of these plants to contain high levels of proteins. They are studying ways to use parts of the plants that can not be eaten and they will also try to find ways of using human wastes to fertilize the -plants. Scientists at the Johnson space Center in Houston, Texas, are trying to grow crops in a mixture of volcanic dirt and crashed glass. The matter is similar to soil found on the moon. Lettuce and wheat have grown well in the mixture. Space Agency scientists hope to test some version of the experimental gardens on the planned space station. But the special growing containers are expected to be used mostly for long distance space travel. A trip to Mars, for example, is expected to take about one year. When the astronauts reach Mars, they could try to use the soil there to grow other crops.