1.C (第一段 But Tower Bridge,the first bridge over the Thames as you travel to London from the sea,is the most famous of them all.)
2.D （第二段 By 1850,everyone agreed that a bridge across the Thames near the Tower was most necessary.）
3.B （第二段 On its north side stands the Tower of London itself.）
4.C （第二段 This took so long because they had two big problems.）
5.A （第二段 The thing that is surprising about Tower Bridge is that it is open in the middle.）