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. Which of the following is the similar problem as that of living in outer space?  (    )
We are short of oxygen. We are not familiar with the underwater surrounding. We may die of weightlessness. We can finds new joys.