棘手的英语句子改错 看来看去好像都对又好像都不对,

棘手的英语句子改错 看来看去好像都对又好像都不对,
While tourists in D.C.they saw the Capitol Building where Congress meets.
a.while
b.the Capitol Building
c.where
d.meets
The bridge at Niagara Falls span the longer unguarded border in the history of the world,symbolizing the peace and goodwill that exsists between Canada and the United States.
a.Niagara Falls
b.the longer
c.exsists
d.between
第二道的C到底对不对，到底要不要改成EXSISTED 或者？EXSIST