When you hear the word "farm", chances are you picture rolling hills in the country covered with cows and cornstalks. Some scientists, engineers and city planners say the farms of the future could rise straight into the air---in skyscrapers in the world's most populated cities.    It might sound far--fetched, but in fact, some of the technology for growing crops indoors already exists. The scientists at the South Pole research station enjoy fresh salads every day from vegetables they grow in their own greenhouse. And NASA has been experimenting for years with methods for growing fresh fruits on the moon or even on Mars.    As is known, traditional farming takes a plot of land as big as the entire state of Virginia just to grow enough food for 8 million people, says ecologist Dichson Despommer. Cities that can't grow their own food are often influenced by disasters such as hurricanes that can make it impossible for trucks to deliver fresh produce to grocery stores. In addition, fruit and vegetables grown outdoors face all kinds of dangers, from flooding to insect pests to weather instability, such as late or early frosts  that can damage a crop. "You can control everything indoors. You can't control anything outdoors," Despommier says.    To top it off, by the year 2050, the world population will grow by another 3 billion. "Where will we grow the food for all these people?" Vertical  farming----growing crops in skyscrapers, tens of stories high----is the answer," says Despommier. Others say  _ could provide new uses for hundreds of abandoned buildings in cities around the world. Crop waste, like leaves, would be composted , and the gases given off from composting would be used to heat the building.    "I'd say it is going to be tough," says agriculturist Giacomelli. It can be tricky to regulate  climate conditions indoors. Plus, plants differ in their weather and lighting requirements. Tomatoes like warm, sunny weather, while greens like lettuce prefer cooler temperatures. And nearly all crop plants require lots of sunlight. From the passage, we can learn that   _  .
scientists haven't developed the technology for vertical farming some of the technology for vertical farming has already been mastered vertical farming will soon replace traditional framing vertical farming is only suitable for big cities
Question: When you hear the word "farm", chances are you picture rolling hills in the country covered with cows and cornstalks. Some scientists, engineers and city planners say the farms of the future could rise straight into the air---in skyscrapers in the world's most populated cities.    It might sound far--fetched, but in fact, some of the technology for growing crops indoors already exists. The scientists at the South Pole research station enjoy fresh salads every day from vegetables they grow in their own greenhouse. And NASA has been experimenting for years with methods for growing fresh fruits on the moon or even on Mars.    As is known, traditional farming takes a plot of land as big as the entire state of Virginia just to grow enough food for 8 million people, says ecologist Dichson Despommer. Cities that can't grow their own food are often influenced by disasters such as hurricanes that can make it impossible for trucks to deliver fresh produce to grocery stores. In addition, fruit and vegetables grown outdoors face all kinds of dangers, from flooding to insect pests to weather instability, such as late or early frosts  that can damage a crop. "You can control everything indoors. You can't control anything outdoors," Despommier says.    To top it off, by the year 2050, the world population will grow by another 3 billion. "Where will we grow the food for all these people?" Vertical  farming----growing crops in skyscrapers, tens of stories high----is the answer," says Despommier. Others say  _ could provide new uses for hundreds of abandoned buildings in cities around the world. Crop waste, like leaves, would be composted , and the gases given off from composting would be used to heat the building.    "I'd say it is going to be tough," says agriculturist Giacomelli. It can be tricky to regulate  climate conditions indoors. Plus, plants differ in their weather and lighting requirements. Tomatoes like warm, sunny weather, while greens like lettuce prefer cooler temperatures. And nearly all crop plants require lots of sunlight. From the passage, we can learn that   _  .
A. scientists haven't developed the technology for vertical farming
B. some of the technology for vertical farming has already been mastered
C. vertical farming will soon replace traditional framing
D. vertical farming is only suitable for big cities
Answer: