One answer to the question of land shortage was suggested by an organization some years ago. A city was to be built at sea, housing 30,000 people. The suggestion was to shape the city like a harbor .The outer wall of the harbor would stand on steel columns resting on the sea-bed. Naturally this could only be where the water was fairly shallow. The people would like to live in flats in the fifty-metre high outer wall. The flats would be all face inwards, and would be made of concrete  and glass. The glass would be specially made and colored to control the heat and strong light from the sun. The planners called this man-made harbor would be calm. On it would be floating islands carrying more building: a hospital, two theaters, museum, an art exhibition hall and a church. On one of the islands would be a special factory to take the salt out of sea water and turn it into fresh water. People living in the city could move around on small boats driven by electricity, so there would be no air pollution from the burning of gas. There would be platforms outside the man wall for ships bringing supplies. People could also travel to the mainland by motorboat or water plane. The suggestion made by the organization is to solve the problem that   _  .
there is not enough space for building cities on land land transportation has became increasingly difficult there are too many people on land present cities are heavily polluted by cars
Question: One answer to the question of land shortage was suggested by an organization some years ago. A city was to be built at sea, housing 30,000 people. The suggestion was to shape the city like a harbor .The outer wall of the harbor would stand on steel columns resting on the sea-bed. Naturally this could only be where the water was fairly shallow. The people would like to live in flats in the fifty-metre high outer wall. The flats would be all face inwards, and would be made of concrete  and glass. The glass would be specially made and colored to control the heat and strong light from the sun. The planners called this man-made harbor would be calm. On it would be floating islands carrying more building: a hospital, two theaters, museum, an art exhibition hall and a church. On one of the islands would be a special factory to take the salt out of sea water and turn it into fresh water. People living in the city could move around on small boats driven by electricity, so there would be no air pollution from the burning of gas. There would be platforms outside the man wall for ships bringing supplies. People could also travel to the mainland by motorboat or water plane. The suggestion made by the organization is to solve the problem that   _  .
A. there is not enough space for building cities on land
B. land transportation has became increasingly difficult
C. there are too many people on land
D. present cities are heavily polluted by cars
Answer: