求英语达人告诉我答案以及为什么选这个答案.要详细一点.

题目

求英语达人告诉我答案以及为什么选这个答案.要详细一点.
Before the first non-stop flight was 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

答案

C.had been 解析：在过去的某个时间段（ the first non-stop flight was made in 1949）之前,发生的事情（all planes to land for refueling.）,即本题考察的是 过去的过去 应该用什么时态A.将来式 .显然不符合B.现...