The Romans built great "aqueducts" to carry fresh water from the mountains to the cities. Many of these aqueducts are still standing today. The Romans also built great pipes under the ground to carry away the sewage. In Rome, one of these sewage pipes(sewers) is still used today; it is 2000 years old. The Roman Emperors even set up government health service. They built the first great public hospitals in Europe, and they paid doctors to look after poor people. Then the Roman Empire fell to pieces, these civilized methods of treatment disappeared from most of Europe, for more than a thousand years. People went back to the old ways. They lived in dirty conditions which helped to cause diseases; and they asked God to cure the diseases. They shut up mentally sick people in prisons. Or they burnt them alive because they were supposed to have magic(有魔力的) powers. But the work of the Greek and Roman doctors was not lost. Over a thousand years ago, the Arabs moved into many of the Mediterranean countries. They took big parts of the old Roman lands. They translated the Greek and Roman medical books into Arabic. Arab doctors themselves made many new discoveries. When civilization at last came back to Europe, men once again translated the Greek and Roman works on medicine into Latin. Slowly—very slowly—European doctors discovered again the things that the Greeks and Romans had known so long ago. Slowly, they began to make new discoveries. They found out more about the way the body works—the way our blood goes round our bodies, the way our nerves send messages from our brains to our muscles, the way these muscles move our bodies. The word "aqueduct" probably means ______.

A. something which was built long ago
B. something invented by a Roman Emperor
C. a big pipe under the ground
D. something built to supply clean water for the citizens