A、They are the strongest bridges in the world. B、They are the oldest bridges in the world. C、They need to be rebuilt very freque

Bridges have been around for an awfully long time. The oldest ones are stone because it’s a very durable material. The Pont Du Gard is a wonderful example—Roman Aqueduct in the south of France—fantastic piece of technology built using massive stones put together, dry—there’s no mortar in those joints.
    Or sometimes up in the mountains, people would build these suspension bridges, often across some dizzy canyon, using a vine. In this case, this is in Peru. This is using grass which grows locally and is woven into ropes to build these bridges. And do you know they rebuild them every year? Because grass is not a durable material. So this bridge is unchanged since Inca times. And bridges can be symbols of their location. Of course, Golden Gate and Sydney are well familiar.
    In Mostar the bridge was synonymous with the name of the place, and to such an extent that in the war in 1993 when the bridge was destroyed, the town all but lost its identity until the bridge was reconstructed. And bridges are enormous features in our landscape—not just enormous, sometimes just a small one, and I believe we have a duty to make our bridges beautiful. Thankfully, many people do. Think of the stunning Millau Viaduct in the south of France. French engineer Michel Virlogeux and British architect Lord Foster collaborated together to produce something which is a really spectacular combination of architecture and engineering. Or Robert Maillart’s Salginatobel Bridge in the mountains in Switzerland—absolutely sublime.
    Or more recently, Laurent Ney’s beautiful and rather delicate bridge for Tintagel Castle in the UK. Bridges can be considered in three convenient categories, depending on the nature of the structural system that they adopt as their principal support. So, bending, of course, is the way a beam will behave—so, beams and bending. Or compression is the principal way of operating for an arch. Or for the really long spans you need to go lightweight, and you’ll use tension, cables—suspension bridges.
    The longest at the moment is in Japan. It’s just under 2,000 meters—one span. The Akashi Kaikyō Bridge. We’re currently working on one in Turkey which is a bit longer, and we’ve designed the Messina Bridge in Italy, which is just waiting to get started with construction one day.
    Question 16. What do we learn about the stone bridges?
    Question 17. What do we learn about Millau Viaduct?
    Question 18. According to the speaker, which country has the longest bridge right now?