925 of 987，多谢！

925        Where once the union had acquiesced to the prejudices of its English-speaking members by supporting the imposition of an alien tax on immigrant workers, after 1897 the United Mine Workers made a determined effort to enlist Italians and Slavs in its ranks.

(A) Where once the union had acquiesced to the prejudices of its English-speaking members by supporting

(B) Where once the union acquiesced to it English-speaking members’ prejudice for the support of

(C) While once the union had acquiesced to the prejudices of its English-speaking members in support of

(D) While once the union acquiesced to its English-speaking members’ prejudice in supporting（A）

(E) While once the union had acquiesced to the prejudices of its English-speaking members in its supporting of

这道题的答案我是比出来的，但是不明白where once在这儿怎么讲。牛人给讲讲吧，多谢！