听力原文： The earth's oceans are colossal, so much bigger than the land area that they cover 70% of the surface of the earth. They give us food and materials and a major constituent in the formation of our weather. The extremes of temperature on the earth would be much greater, and many places would be too hot or too cold for humans without the oceans to act as the earth's thermostat. As our supply of fresh water becomes more and more scarce, we have naturally looked to the oceans, the greatest source of water on earth. But there is so much salt in sea water that it is not fit for human consumption. Scientists have been trying to find ways to turn this salt water into fresh water. Machines for the conversion of water are already at work in certain parts of the world, but their cost is still exorbitant. Science will have to find much cheaper ways to do the job. Possibilities for food abound in the ocean. In addition to the hundreds of varieties of fish, some kinds of seaweed can be eaten. It is nutritious enough, containing many proteins, vitamins, and minerals that humans need for survival, but so far no one has been able to figure out how to disguise its awful taste. (30)