XDF 补充教材 section 23-18

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 memebers by  supporting

B. Where once the union acquiesced to its 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' prejudece in supporting

E. While once the union had acquiesed to the prejudices of its English-speaking members in its supporting of

Why A not D?

Where once 是什么意思？ 有while once 吗？谢谢！！！

D.  While once the union acquiesced to its English-speaking members' prejudece in supporting

Wrong tense.

thank you rhod