21.
(A) The other
(B) Other
(C) Others
(D) Another

One…;another…; the other…

But the cause that makes people    19    in London is language, because they want their children to build up the    20    in English, which is a great gift and advantage in the global context.    21    cause is surprising. Londoners tend to leave people    22   , and this gives newcomers much space to live their own lives. Finally, food is also an important cause bringing people all over the world to London.
文章提到三個原因，這題是第二個原因，所以用another。the other是限定用法，特別指出就是那個，所以要放最後面。