Can you imagine traveling to work in a one - man submarine? Some scientists believe that some day one - man submarines will be as many as automobiles are today.A famous French driver says, "One day soon, men will walk on the ocean floor as they do on the street!" Perhaps during your lifetime people will travel, and live in the sea. If human beings want to live in the ocean, many human problems will need to be studied first. Some of these problems, similar to those of living in outer space, are pressure, lack of oxygen and weightlessness. Many questions are still unanswered.For example, can our blood make itself fit for underwater surroundings? What will happen to our muscle if we live in the water very long? Scientists are looking for answers. Perhaps in the future man will live in the sea, away from the crowded and noisy cities on land.Then sea has plenty of space, not only for floating living buildings and parks, but also for storing supplies and for underwater travel. Some scientists believe that ocean living will benefit man in more than physical ways. In the freedom and beauty of the deep sea, man may find new sources of joy. Why do some people hope to leave cities to live in the sea? Because  _  .  (   )
people think they can live crowdedly in the sea people wish to go the quiet seafloor to travel for several days people want to break away from the crowded and noisy cities where they live now only in this way can people get rid of noise pollution