题目内容：
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
参考答案：
选择题：Before the first nonstop flight made in 1949, it
- 题目分类：语言/文学
- 题目类型：选择题
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