2. 专升本
  3. Before the first non-stop flight made in 1949, it______ necessary for all planes to land for refueling.

Before the first non-stop flight made in 1949, it______ necessary for all planes to land for refueling.

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问题 Before the first non-stop flight made in 1949, it______ necessary for all planes to land for refueling.
选项 A、would be
B、has been
C、had been
D、would have been
答案C
解析 句意：1949年进行直航之前，所有飞机都必须降落加油。本题考查时态的应用。句中由before引导的时间状语从句表示句中的动作是发生在过去(1949年)之过去，那么主句就应该使用过去完成时。所以选项C为正确答案。
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