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 fifty-metre high outer wall. The flats would 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 water the lake. The water inside this man-made harbor would be calm. On it would be floating islands carrying more buildings; a hospital, two theaters, a 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 main wall for ships bringing supplies. People could also travel to the mainland by motorboat or water plane.
What was suggested about the construction of a new city at sea？

A. The city was to be designed together with a harbor.
B. The walls around the city would be made of steel and glass.
C. The buildings of the city would rest on a floating island.
D. The people would live in tall buildings surrounded by a wall.