一道语法题，请大家指点

NKspielberg

白勇语法P128第18题

Once the economic and social usefulness of the motor car was demonstrated and (with its superiority to the horse being )proved, much of the early hostility to it in rural regions disappeared. 

B. and its superiority over the horse had been

C. and its superiority to the horse

白勇的答案是B。但我却认为应该是C。我的想法：C承前省略了was，was proved 和 was demonstrated 构成很好的并列。而B的had been用过去完成时却没有道理。

NKspielberg

自己顶一下