Where do you think people will live in the future? Can you predict people will work and live in the sea? But how will people go to work then? By submarine! Some scientists believe that some day submarines will be as many as today's cars. A famous French driver says, " One day, man will work on the ocean floor as they do on the street!" If people want to live in the sea, they have to solve a lot of problems first. Some of these problems, similar to those of living on the moon, are lack of oxygen   and weightlessness  . Many questions are hard to answer. For example, what will happen to our bodies if we live in the sea for a very long time? Scientists are looking for answers. Maybe in 50 years man will be able to live in the sea, away from the crowded and noisy cities on land. Man will find it fun and interesting to live in the sea. And life in the sea is more comfortable. Don't you think so? People have to   _   first before living in the sea.
build houses learn to swim learn to drive the submarine solve some problems