Before the first non-stop flight made in 1949, it ____ necessary

Before the first non-stop 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
【正确答案】：C
【题目解析】：根据before引导的时间状语从句可知，所有飞机都必须着陆加油发生在实现直飞之前，所以主句要用过去完成时，故选C。句意：在1949年首次实现直飞之前，所有飞机都必须着陆加油。