Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Scientists have dreamed for a long time about building a 'flying train' which can float through the air above the tracks. With Maglev technology, their dreams are now a reality. Maglev trains have no wheels. Instead, powerful magnets lift them into the air and push them forward, at speeds of up to 500 km per hour.

As well as speed, Maglev has a number of advantages over other transport systems. The trains are quieter and the journey is smoother because they don't have wheels. They are also greener because they don't use fuel and so create less pollution. Right now there is only one high-speed Maglev train system in the world. It carries passengers from Shanghai city centre to the airport. However, work has started on a new, longer Maglev track in Japan. Trains will go from Tokyo to Nagoya in 40 minutes. But don't expect to ride on it on your next trip to Japan. It won't open until 2027! Then, in 2045, the line will extend to Osaka.

There is one major problem with Maglev technology: it is incredibly expensive to build the tracks. Transport planners predict that the Tokyo to Osaka line will cost 91 billion dollars. The high cost means that Maglev technology probably won't replace other forms of transport in the near future. However, the technology is still in development and somebody might find a cheaper and simpler way to build the tracks. And then, who knows? In the future, some of us might fly to work by train.

(Adapted from Navigate by Caroline Krantz and Julie Norton)

The word “they” in paragraph 2 refers to ______.