Without transportation, there could be no trade. Without trade, there could be no towns and cities. Towns and cities are traditionally the ___16___ of civilization. Therefore, transportation helps make civilization ___17___ .  Throughout most of the prehistoric period, people lived by hunting, fishing, and ___18___ wild plants. They had no beasts of burden, wheeled vehicles, or roads. People traveled ___19___ and carried their infants and belongings strapped to their backs or heads. Loads too heavy for one person to carry were strapped to a pole and carried by two people.  ___20___ , people learned that they could drag loads along the ground on sledges. During late prehistoric times, people began to build sledges with runners. When people developed agriculture and began to establish ___21___ settlements, trade between settlements  started to develop, which created a need for better ___22___ of transportation. The donkey and the ox, which had been tamed for food products and farm work, helped ___23___ this need. The use of donkeys and oxen as beasts of burden enabled people to ___24___ heavier loads than they could before.  People also began to develop water transportation during prehistoric times. They built rafts of such __25____ as logs or reeds. Later, people learned ___26___ to make dugouts and canoes. People paddled these early craft with their hands  or propelled them with paddles or __27____. The wheel was invented about 3500 B.C. and sailboat about 3200 B.C. Wheeled vehicles and sailing vessels ___28___transportation. But the speed of transportation improved only ___29___ over the centuries. Inventors produced the first engine- ___30___ vehicles during the late 1700’ s and the early 1800’ s. This development marked the beginning of a revolution in transportation that has continued to the present.  16. A. highlights B. pinpoints C. headquarters D. centers 17. A. possible B. acceptable C. practical D. typical 18. A. growing B. gathering C. watering D. moving 19. A. in turn B. at ease C. on foot D. for fun 20. A. All the while B. For a while C. In time D. On time 21. A. temporary B. permanent C. regular D. steady 22 A. knowledge B. judgment C. standards D. means 23. A. recognize B. develop C. meet D. reduce 24. A. bear B. move C. share D. lift 25. A. materials B. styles C. items D. types 26. A. where B. when C. how D. as 27. A. poles B. sails C. currents D. winds 28. A. demonstrated B. revolutionized C. witnessed D. initiated 29. A. considerably B. continuously C. swiftly D. slightly 30. A. powered B. controlled C. oriented D. related