阅读理解。 America hopes to start building a base that can continue to exist all the time on the moon within 20 years. By the mid-2020s, astronauts will be staying there for six months a time. So what do scientists think food and water will be like for those first moon residents? In the short term, food would have to be brought from Earth. The dream, however, is to grow things in greenhouses on the moon. 'You'd grow things in the liquid-there's no reason why that wouldn't work,' says Pillinger, a professor. This would mean floating plants in a nutrient-rich solution of water. 'Soil is not a necessary thing for plants; it's only to keep them standing upright,' Pillinger adds. It would take at least a decade of experiments after the base was built before such a plan would work, however. The main problem is working out how to get nutrients from the lunar soil. 'Getting all that to work requires a minimum level of infrastructure (基础设施) present before you can start experimenting with greenhouses,' says Pillinger. Finding strong plants to grow in space will also be hard, though scientists are already working on it: scientist Amy Grunden of North Carolina State University has been working on genetically engineering food crops that can be grown in harsh, off-planet environments. Water is a somewhat easier prospect (前景). One of the reasons that America wants to put its base at the moon's south pole is that it thinks that, in the permanently shaded craters, there are large pools of frozen water. If that's true, it could easily be mined and used to drink and to create oxygen for the habitants. 1. The passage is mainly about _____. A. life on the moon by 2020 B. America's first moon base that exists forever C. food and water on the future moon base D. housing for settlers on the future moon base 2. How do people plant on the moon in the future? A. By burying plants in the lunar soil. B. By growing plants in water. C. By keeping plants standing straight. D. By making use of the moonlight. 3. What difficulty will scientists meet in solving the food problem? A. How to find water to irrigate plants. B. Where to experiment with plants. C. How to grow plants in the moon soil. D. What plants are suitable to grow on the moon. 4. Scientists want to build the base on the moon's south pole because _____. A. there is oxygen for people to breathe B. people have found the frozen water that can be drunk there C. it is a place where water will possibly be found and supplied D. there are large pools that can let people have a swim