请教一道白勇题目！

yangxu1234

Once the economic and social usefulness of the motor car was demonstrated {and with its superioty to the horse beinig }proved,much of the early hostility to it in rural regions disappeared.
(A) and with its superioty to the horse beinig
(B) and its superioty over the horse had been
(C) and its superioty to the horse
(D) its superioty over the horse
(E) with its superioty to the horse having been

once从句里的主语应该是the economic and social usefulness 为什么谓语动词用was 不用复数  还有后面的its指代的是什么？这道题应该选B还是C呢  我在网上看有人说C是省略 但白勇又说是Once从句并列 我越读约迷惑了  求教

salasbond

这道题算C，主语是usefulness所以谓语是单数，its指代的是car，这道题的确是省略，省略了was：and its superioty to the horse (was) proved