A.a century after its construction
B.because of large weights in the area
C.during its construction
D.because of its stone structure
参考资料
Engineers are proud of the completion of repairs to the Leaning Tower of Pisa(比萨塔).Experts think the repairs will make the building safe for at least a few more centuries, but now only about thirty visitors are permitted to climb the tower at one time.The Leaning Tower is famous around the world.Many visitors to Italy travel to the city of Pisa to see the unusual-looking structure.The round tower is more than 800 years old.It is easy to recognize.The white stone building is sinking,or leaning, on one side.The tower is about 55 meters high.Its south side is much lower than its north side.The Tower i leaning because it was built on soft sandy soil.The soil cannot support its weight.The tower weighs about 14,500 metric tons.The ground under the tower first started to sink soon after workers began building the structure in the year 1173.In recent decades, the Tower continued to sink.Experts expressed concern that the tower might fall suddenly.That's why Italian officials closed the Leaning Tower of Pisa in 1990.An international team of building experts began a project to correct the slant.The project cost about 25 million dollars.First, engineers placed 870 metric tons of lead weights on the north side of the tower.This helped stop additional movement.Also, workers tied strong steel cables around the structure.These wires were connected to large weights in the area.Next, engineers slowly removed tons of soil from under the tower.They used special drills to remove small amounts of soil over several months.Slowly the tower moved back to where it stood hundreds of years ago.It was reopened to the public in 2000.