Question 19 is based on the recording you have just heard.

Recording Two

听力原文

    (19) We’ve talked recently about the importance of sustainable energy. We’ve also talked about the different theories on how that can be done. So far, our discussions have all been theoretical. Now I have a practical question for you all. (20) Can you run a 140,000-kilogram train on just the steam generated by solar power? Well, one engineer, Tim Casselman, believes it’s possible.

    And his home city of Sacramento, California, should see the technology’s first test. As part of the upgrading of its rail yard, Casselman, who is an inventor and self-proclaimed steam visionary, is campaigning for a new steam train that runs without any fire and could run on an existing 10-kilometer line, drawing tourists and perhaps offering city commuters a green alternative to their cars. Casselman wants to build an array of solar magnifying mirrors at one end of the line to collect and focus heat onto water-filled tubes.

    This would generate steam that could be used to fill tanks on a small steam train without the use of fire. “Supplying power to trains in this way would offer the shortest distance from well to wheels,” he says, “with the least amount of energy lost.” According to Harry Valentine, a Canadian engineer who is researching modern steam technology, a special tank measuring 2 by 10 meters could store over 750 kilowatt hours of energy as high pressure steam enough to pull a two-car train for an hour or so.

    Energy to drive a steam locomotive can be stored in other materials besides water. For example, a team at Tohoku University in Japan has studied materials that can store large amounts of heat. When heated, these materials turn from a solid into a liquid, absorbing energy as they change phase. The liquid is maintained above its melting point until steam is required, at which point the liquid is allowed to turn back into a solid, releasing its stored energy. (21) Another team at Nagoya University in Japan has tested calcium compound as an energy storage material, heating this chemical compound drives off carbon dioxide gas, leaving calcium oxide.

    The gas can be stored under pressure in a tank. To recover the energy, the gas is bent back over the calcium oxide. In theory, says Valentine, this can create a high enough temperature to generate superheated steam. 

19. What has the speaker previously talked about?

解析：D。本文开头中，讲话者说到他之前已经提到了可持续能源的重要性以及如何实现能源可持续的各种理论。原文第二句中的that就是指第一句中的sustainable energy。因此本题选D。