单项选择题

From the very beginning, water has furnished man with a source of food and a highway to travel. The first civilizations arose (76) water was the dominant element in the environment, a challenge (77) man’s ingenuity. The Egyptians invented the 365-day calendar (78) the Nile’s annual flooding. The Babylonians (79) were among the most famous law-makers in ancient times, invented laws (80) water usage. Water inspired the Chinese to build a 1,000 miles’ canal, a complex system (81) , after 2,500 years, remains particularly in use and still commands the astonishment of engineers. But the ancients never found complete solutions (82) their water problem. The Yellow River is also known as "China’s Sorrow". It is so unpredictable and dangerous (83) in a single flood it has caused a million deaths. Floods slowed the great (84) of the Indus River Valleys, and innumerable damages ruined much of its land. Today, water dominates man as it always has done. Its presence continues to influence the location of his homes and cities; its violent variability can (85) man or his herds or his crops; its routes link him to his fellows; its immense value may add to the already dangerous political conflicts. There are many examples of this in our own time.