请问大全925

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

答案是A, 有一点不明白，请问:为什么要用Where once? 是固定搭配，或者有什么特定含义？