[单项选择]The history of civil engineering is a very important story in the development of civilization. Civil engineers began practicing their (47) four thousand years ago. Every great civilization, Egyptian, Minoan, and Roman, (48) , had their civil engineers. What is left of their work is proof that they were intelligent. These engineers of the ancient world built entire cities. They (49) systems of pipes which supplied fresh water. They built water pipelines for farm lands as well as bridges of great length. The ruins of their structures tell us (50) about the work of early engineers. Consider the pyramids of Egypt. They are (51) that some of the earliest engineers had great scientific ability. They are also proof that those engineers could make lasting works of art and design. Although they lived thousands of years ago, the Egyptian engineers used very (52) measurements. The base of the largest pyramid -- the great pyramid near Gizeh -- comes to within inches of being a perf