正确答案： A

解析：
[考点]细节题。录音提到，若用冷海水，整个美国西海岸将拥有空调的功能。这是冷海水的一个用途。
【听力原文】
　　Do you know that vegetables can be grown in climates they are not accustomed to? Cool climates vegetables are now able to be grown in places as hot as Hawaii.
　　In Hawaii, engineers have been able to actually make those vegetables live in cooler climates. That way they grow faster and taste better. What these engineers have been using is cold seawater. The temperature of seawater is much lower than the air temperature.
　　[41] How do they use it? They place pipes in the soil and the cold water flowing through them cools the earth. This stimulates plant growth and enables gardeners in tropical climates to grow crops from cooler climates.
　　Also, some of these pipes are exposed to the air and they make the moisture form little water drops and thus irrigate the gardens. What is especially appealing about this process is that nothing damaging to the natural environment is being used.
　　[42] Another innovative use for cold ocean water is to cool buildings. Engineers believe that, for example, [43] the entire west coast of the United States could be air-conditioned using seawater.
　　We all know there are certain types of chemicals used in air conditioner, which not only pollutes the air but also bring damage to the ozone. If we could cool the city by using seawater, the environment of the whole city will be improved and the ozone will be protected, which will be beneficial to ourselves in return.
Questions 40 to 43 are based on the passage you have just heard.
40. What is the report mainly about?
41. How are the pipes used in Hawaii?
42. What are the other advantages of using seawater?
43. Why does the speaker mention air-conditioning?