Question

Before the first nonstop flight made in 1949,it______necessary for all planes to land for refueling.

A．would be

B．has been

C．had been

D．would have been

Answer 1

C

本题时间状语从句为一般过去时，主句的情况是在从句之前，是“过去的过去”，因此用过去完成时。