A Train Floating On Air A train that floats on air? It's not magic--it's magnets  .And it's close to reality. In Virginia USA the fall of 2002, a train with no wheels traveled on air and carried college students across their campus.In Japan, a whisper-quiet railway engine hovered and raced at 350 miles per hour using magnets and electricity as the power.And in China, a magnet train line linked Shanghai with nearby Pudong Airport. These trains use magnetic levitation   technology, "maglev" for short. They use the same rules as the magnets you pick up at home or school: opposite poles of magnets attract each other, and like poles _ each other. How does it work? Powerful magnets on the bottom of the train repel magnets on the track, which is actually just a magnet-filled guiding way. With a magnetic field of sufficient force, the train will go hovering on air, which seemed impossible to us in the past. When an electrical current is sent through the track, the train moves. Turn the current backwards and the train slows down. Maglev doesn't rely on the friction   of wheels on track, so it can climb a much steeper hill than a traditional train. And it can travel easily in snow and ice, something that could bring normal trains to a screaming stop. This passage is about   _  .
maglev magnets levitation electricity
Question: A Train Floating On Air A train that floats on air? It's not magic--it's magnets  .And it's close to reality. In Virginia USA the fall of 2002, a train with no wheels traveled on air and carried college students across their campus.In Japan, a whisper-quiet railway engine hovered and raced at 350 miles per hour using magnets and electricity as the power.And in China, a magnet train line linked Shanghai with nearby Pudong Airport. These trains use magnetic levitation   technology, "maglev" for short. They use the same rules as the magnets you pick up at home or school: opposite poles of magnets attract each other, and like poles _ each other. How does it work? Powerful magnets on the bottom of the train repel magnets on the track, which is actually just a magnet-filled guiding way. With a magnetic field of sufficient force, the train will go hovering on air, which seemed impossible to us in the past. When an electrical current is sent through the track, the train moves. Turn the current backwards and the train slows down. Maglev doesn't rely on the friction   of wheels on track, so it can climb a much steeper hill than a traditional train. And it can travel easily in snow and ice, something that could bring normal trains to a screaming stop. This passage is about   _  .
A. maglev
B. magnets
C. levitation
D. electricity
Answer: