A、Prehistoric times. B、As early as 3200 B.C. C、Roman times. D、Around A.D. 550. A细节题。文中提到，人们从史前时期(prehistoric times)就开始建造桥梁了，故答案为

This is the first of a series of lectures on historic engineering structures. Today, we’re looking at the Clifton Suspension Bridge in Bristol, which we hope to visit later this term, and I’d like to begin with a brief word about the bridge’s history and about bridge building in general.[19]Well, people have been building bridges since prehistoric times. Over the centuries, bridge design has evolved using a variety of engineering techniques, but the objective has always been the same: to get to the other side.
     [20]One of the most basic types of bridge is the arch, and there’s evidence from the Middle East that people knew how to construct arches using stone or brick as early as 3200 B.C. The stone arch had the advantage of being quite simple to build, and it remained the main type of bridge design from Roman times until the early 1700s. Another type of bridge with a long history is the suspension bridge, where the road is suspended from cables hanging between towers.[21]The first suspension bridges were simple affairs, made of rope and wood, and the earliest recorded examples were constructed around A. D. 550 in China. But rope has limited strength, and it only became possible to build longer bridges when iron became available. The first major iron suspension bridge in Europe, completed in 1826, was the Menai Strait Bridge in Wales.
19. When could building bridges date back to?
20. Which of the following is mentioned as one of the most basic types of bridge?
21. Where is the earliest recorded suspension bridge located?