做GWD的困惑

lllmmm

A March 2000 Census人口普查 Bureau survey showed that Mexico accounted for more than a quarter of all foreign-born residents of the United States, ************************* since 1890, when about 30 percent of the country’s foreign-born population was from Germany.

A.    the largest share for any country to contribute
B.    the largest share that any country has contributed
C.    which makes it the largest share for any country to contribute
D.    having the largest share to be contributed by any country
E.    having the largest share to have been contributed by any country
我知道答案肯定在A和B中   为什么选B不选A呢

twobirds

since是个完成时态的标志词

sheikh239

for sb to do sth 放在这里不知道想说明什么意思

为了让某人做某事？