A、It is already at work all over the world. B、It costs too much. C、Scientists have no idea about how to transform it. D、There is

The earth’s oceans are huge, so much bigger than the land area that they cover 70% of the surface of the earth. Throughout history humans have been directly or indirectly influenced by the oceans. (4) Ocean waters serve as a source of foods and valuable minerals. Increasingly, people are turning to the oceans for their food supply either by direct consumption or indirectly by harvesting fish. Besides, oceans are a major element 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. 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 seawater that it is not fit for human use. Scientists have been trying to find ways to turn this salty water into fresh water. (5) Machines for the transformation of water are already at work in certain parts of the world, but their cost is still much too high. Scientists will have to find much cheaper ways to do the job. It is possible that there is plentiful food in the ocean. In addition to the hundreds of varieties of fish, some kind of seaweed can be eaten. (6) It is beneficial 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 awful taste.
4. What can the ocean supply us with?
5. What do we know about the transformation of seawater into fresh water?
6. What is the disadvantage of seaweed as food?