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

答案是B，但是如果我把and ……看作是和once……demonstrated并列的，那么在并列句里是一定要省略Be的，所以我觉得C是真确的。大家的意见呢？