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阅读下列短文，从每题所给的四个选项A、B、C、D中，选出最佳选项。
The tower of Big Ben,London's most famous building,has always leaned（倾斜）slightly,but construction work on a new line for the London Underground seemed likely to give the tower a real lean．Engineers have had to prop up（支撑）its base to prevent it from damaging the rest of the Houses of Parliament．
To control the tower's movement,engineers pumped grout（水泥浆）into tile soil under the tower．The tower now leans an extra few centimetres,but the lean can only be seen by the most sharp— eyed observer．
Between 1995 and 1997,to lengthen the Jubilee line of the Underground,builders dug a 40．— metre— deep hole just 31 metres north of the clock tower．And the new tube(underground)tunnels' were even nearer to the tower．John Burland,who recently helped stop the increasing lean in the Leaning Tower of Pisa and was an adviser to the Jubilee line project,believed the work would affect the tower．In his opinion,no further lean should go beyond a safety limit of 27.5 millimetres over existing lean of 220．millimetres．
To keep the lean within this limit,Burland told the British Association about the new method of pumping grout immediately under the base of the tower．More than 300 tons of grout were pumped in during construction．The tower's additional lean went between 10．and 25 millimetres,but never passed 27.5 millime_Ires．
After the construction work was completed in late 1997,a review of movements in nearby walls showed that the building was in better shape than had been thought before and the safety limit was raised to 35 millimetres before any action needs to be taken．
Since 1997,the tower has continued to lean,Burland told Modern Construction．The latest measurements,taken this year,suggest that the tower's lean had just reached 35 milllmetres．But following regular re— examinations of the tower,experts are sure that the tower has stabilised（稳定）．A spokesman for London Underg-round says:“We understand the tower has stabilised and returned to its normal movement cycle．”
From Modern Construction,16 September,2000．
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