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。